DataFrame initial support (#173)

* initialize dataframe

* adjust inheritance for mars/core and mars/serialize, serialize no longer inherit from mars/core, and SerializableWithKey and AttributeAsDictKey has been moved to mars/core

* move Chunk and ChunkData to mars/core

* refine import and comment

* add dataframe.proto

* add TilesableData, make TensorData inherit from it

* add SliceField

* add TilesableOperandMixin

* add new_chunks in the TilesableData, add TensorChunkData inherited from ChunkData and TensorChunk inherited from Chunk

* add new_entities

* add IndexChunkData

* add chunk type of index, series and dataframe

* add DataFrameOperandMixin

* add DataFrame datasource

* sync rough_bytes

* add decide_chunk_sizes for dataframe

* support tokenize type relative to pandas

* finish expression part of from_pandas

* move executor to top level, add dataframe execute support

* support serialize dataframe for graph

* refine ut

* fix according to codacy

* remove unused code to increase coverage

Author: qinxuye

.travis.yml

@@ -65,10 +65,10 @@ before_script:
 script:
   mkdir -p build &&
   pytest --cov-report= --cov-config .coveragerc-tensor --cov=mars --timeout=1500 -W ignore::PendingDeprecationWarning
-    mars/tensor mars/web &&
+    mars/tensor mars/dataframe mars/web &&
   mv .coverage build/.coverage.tensor.file &&
   pytest --cov-report= --cov-config .coveragerc --cov=mars --timeout=1500 --forked -W ignore::PendingDeprecationWarning
-    --ignore mars/tensor mars &&
+    --ignore mars/tensor --ignore mars/dataframe mars &&
   mv .coverage build/.coverage.main.file &&
   coverage combine build/ && coverage report --fail-under=85 &&
   export DEFAULT_VENV=$VIRTUAL_ENV &&