SF: Add documentation for avg in group mode and extract operation

to rst and SF help notebook

Author: MichaelHuth

Packages/MIES/SweepFormulaHelp.ifn

@@ -332,11 +332,15 @@ avg and mean
 `avg` and `mean` are synonyms for the same operation.
 They calculate the arithmetic average :math:`\frac{1}{n}\sum_i{x_i}`.
 
-data: input data wave(s)
+data: input data wave(s) or array of datasets
 
 mode: optional parameter that defines in which direction the average is applied.
       - `in` default, applies the average over each input data wave. In this mode the operation returns the same number of waves as input waves were specified. Each output wave contains a single data point. If input data type is `SF_DATATYPE_SWEEP` from the data operation the sweep meta data is transferred to the returned data waves. The default suggested x-axis values for the formula plotter are sweep numbers.
       - `over` averages over all input data waves. In this mode the operation returns a single wave. `NaN` values in input waves are ignored in the average calculation. A trace generated from the returned wave will be shown as topmost trace in the default color for averaged data.
+      - `group` accepts an array of datasets as the first argument where each dataset must have the same number of elements. The average calculation is performed over the n-th elements of each dataset. Example: Lets say one has three datasets of sweep data with sweep 1, 2, 3; 4, 5, 6 and 7, 8, 9. Each prepared in an own variable `sweepset0`, `sweepset1` and `sweepset2`. When `avg([$sweepset0, $sweepset1, $sweepset2], group)` is called the averaging works on sweep 1, 4, 7, then 2, 5, 8 and finally 3, 6, 9. The result is a new dataset of the same size (three) as the input datasets. The input datasets can be of any type and do not need to be sweep data. The first argument must contain at least two datasets. Meta data (if present) is transferred from the first element datasets in the array to the result.
+
+When an argument is of the select type then automatically data is applied to implicitly convert to sweep data.
+This conversion is also applied in the `group` mode for array element that are of select type.
 
 The returned data type is `SF_DATATYPE_AVG`.
 
@@ -348,6 +352,21 @@ The returned data type is `SF_DATATYPE_AVG`.
 
    avg(data(select(selrange(ST))), in)
 
+   avg([select(selrange(E1)), select(selrange(E2))], group)
+
+extract
+"""""""
+
+.. code-block:: bash
+
+   extract(datasets, index)
+
+`extract` takes datasets as first argument and an index number as second argument. It returns the dataset from position index.
+
+.. code-block:: bash
+
+   sweep2DA = extract(data(select(selsweeps(1, 2, 3), selvis(all), selchannels(DA))), 1)
+
 root mean square
 """"""""""""""""