docs update and getting ready for release

Author: v0dro

README.md

@@ -340,7 +340,6 @@ Head over to the tutorials and notebooks listed above for more examples.
 * Basic Data manipulation and analysis operations: 
     - Different kinds of join operations
     - Dataframe/vector merge (left, right, inner, outer)
-    - Creation of correlation, covariance matrices
     - Verification of data in a vector
     - DF concat
 * Option to express a DataFrame as an NMatrix or MDArray so as to use more efficient storage techniques.

lib/daru.rb

@@ -3,6 +3,7 @@ def jruby?
 end
 
 require 'csv'
+require 'matrix'
 require 'securerandom'
 
 require 'daru/index.rb'
@@ -12,4 +13,3 @@ def jruby?
 require 'daru/monkeys.rb'
 
 require 'daru/core/group_by.rb'
-

lib/daru/accessors/nmatrix_wrapper.rb

@@ -1,4 +1,8 @@
-require 'nmatrix' unless jruby?
+begin
+  require 'nmatrix' unless jruby?
+rescue LoadError => e
+  puts "Please install the nmatrix gem for fast and efficient data storage."
+end
 
 module Daru
   module Accessors

lib/daru/core/group_by.rb

@@ -30,6 +30,14 @@ def size
         Daru::Vector.new(values, index: index, name: :size)
       end
 
+      def first
+        head(1)
+      end
+
+      def last
+        tail(1)
+      end
+
       def head quantity=5
         select_groups_from :first, quantity
       end

lib/daru/dataframe.rb

@@ -513,13 +513,53 @@ def sort vector_order, opts={}
 
     # Pivots a data frame on specified vectors and applies an aggregate function
     # to quickly generate a summary.
+    # 
+    # == Options
+    # 
+    # +:index+ - Keys to group by on the pivot table row index. Pass vector names
+    # contained in an Array.
+    # 
+    # +:vectors+ - Keys to group by on the pivot table column index. Pass vector
+    # names contained in an Array.
+    # 
+    # +:agg+ - Function to aggregate the grouped values. Default to *:mean*. Can
+    # use any of the statistics functions applicable on Vectors that can be found in 
+    # the Daru::Statistics::Vector module.
+    # 
+    # +:values+ - Columns to aggregate. Will consider all numeric columns not 
+    # specified in *:index* or *:vectors*. Optional.
+    # 
+    # == Usage
+    # 
+    #   df = Daru::DataFrame.new({
+    #     a: ['foo'  ,  'foo',  'foo',  'foo',  'foo',  'bar',  'bar',  'bar',  'bar'], 
+    #     b: ['one'  ,  'one',  'one',  'two',  'two',  'one',  'one',  'two',  'two'],
+    #     c: ['small','large','large','small','small','large','small','large','small'],
+    #     d: [1,2,2,3,3,4,5,6,7],
+    #     e: [2,4,4,6,6,8,10,12,14]
+    #   })
+    #   df.pivot_table(index: [:a], vectors: [:b], agg: :sum, values: :e)
+    # 
+    #   #=> 
+    #   # #<Daru::DataFrame:88342020 @name = 08cdaf4e-b154-4186-9084-e76dd191b2c9 @size = 2>
+    #   #            [:e, :one] [:e, :two] 
+    #   #     [:bar]         18         26 
+    #   #     [:foo]         10         12 
     def pivot_table opts={}
       raise ArgumentError, "Specify grouping index" if !opts[:index] or opts[:index].empty?
 
       index   = opts[:index]
       vectors = opts[:vectors] || []
       aggregate_function = opts[:agg] || :mean
-      values  = opts[:values] ? [opts[:values]] : ((@vectors.to_a - (index | vectors)) & numeric_vectors)
+      values = 
+      if opts[:values].is_a?(Symbol)
+        [opts[:values]]
+      elsif opts[:values].is_a?(Array)
+        opts[:values]
+      else # nil
+        (@vectors.to_a - (index | vectors)) & numeric_vectors
+      end
+      
       raise IndexError, "No numeric vectors to aggregate" if values.empty?
 
       grouped  = group_by(index)

spec/fixtures/sales-funnel.csv

@@ -0,0 +1,18 @@
+Account,Name,Rep,Manager,Product,Quantity,Price,Status
+714466,Trantow-Barrows,Craig Booker,Debra Henley,CPU,1,30000,presented
+714466,Trantow-Barrows,Craig Booker,Debra Henley,Software,1,10000,presented
+714466,Trantow-Barrows,Craig Booker,Debra Henley,Maintenance,2,5000,pending
+737550,"Fritsch, Russel and Anderson",Craig Booker,Debra Henley,CPU,1,35000,declined
+146832,Kiehn-Spinka,Daniel Hilton,Debra Henley,CPU,2,65000,won
+218895,Kulas Inc,Daniel Hilton,Debra Henley,CPU,2,40000,pending
+218895,Kulas Inc,Daniel Hilton,Debra Henley,Software,1,10000,presented
+412290,Jerde-Hilpert,John Smith,Debra Henley,Maintenance,2,5000,pending
+740150,Barton LLC,John Smith,Debra Henley,CPU,1,35000,declined
+141962,Herman LLC,Cedric Moss,Fred Anderson,CPU,2,65000,won
+163416,Purdy-Kunde,Cedric Moss,Fred Anderson,CPU,1,30000,presented
+239344,Stokes LLC,Cedric Moss,Fred Anderson,Maintenance,1,5000,pending
+239344,Stokes LLC,Cedric Moss,Fred Anderson,Software,1,10000,presented
+307599,"Kassulke, Ondricka and Metz",Wendy Yule,Fred Anderson,Maintenance,3,7000,won
+688981,Keeling LLC,Wendy Yule,Fred Anderson,CPU,5,100000,won
+729833,Koepp Ltd,Wendy Yule,Fred Anderson,CPU,2,65000,declined
+729833,Koepp Ltd,Wendy Yule,Fred Anderson,Monitor,2,5000,presented