ensure mutation feeds back into the grouped.obj

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• Ensure mutation on the dataframe is passed to the groupby
• The above ensures subsequent calls within the mutate method can be applied to just created columns, that were probably not existing
• e.g, if a column does not exist in the grp.obj columns, this PR ensures everything works fine.
  df.groupby('rar').mutate({'a':('b','sum'), 'a','prod'})

Please tag maintainers to review.

• @ericmjl

[ericmjl]

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