fix #37 set copy warning

[JanBenisek]

Fixes warning A value is trying to be set on a copy of a slice from a DataFrame. Try using .loc[row_index,col_indexer] = value instead

The issue was actually in how Cobra is used, not the code. In the documentation, we do the following:

# fit the pipeline (will automatically be stored to "path" variable)
preprocessor.fit(basetable[basetable["split"]=="train"],
                 continuous_vars=continuous_vars,
                 discrete_vars=discrete_vars,
                 target_column_name=target_column_name)

You notice that first parameter, where we are passing slice of a dataframe. When we further operate on that dataframe (adding features), the warning is triggered, since we are working with a slice.

Solution
Enforce .copy() of the dataframe before we work with it, inside cobra.

Source of the warning: https://stackoverflow.com/questions/20625582/how-to-deal-with-settingwithcopywarning-in-pandas

[sandervh14]

OK!