adding documentation for pyagg helper

doc/04_pyAgg.md

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+## PyAgg Helper Function 
+
+### CDB_pyAgg (columns Numeric[])
+
+Currently it's not possible to pass a multidiemensional array between plpsql and plpythonu. This function aims to
+help fix that by aggergating the columns provided in the argument across rows in to a rows * columns + 1 length 1D array. The first element of the array is the array\_length of the columns argument so that python can reconstruct 
+the 2D array. 
+
+#### Arguments
+
+| Name | Type | Description |
+|------|------|-------------|
+| columns | NUMERIC[] | The columns to aggregate across rows|
+
+#### Returns
+
+A table with the following columns.
+
+| Column Name | Type | Description |
+|-------------|------|-------------|
+| result | NUMERIC[] | An columns * rows + 1 array where the first entry is the no of columns|
+
+