Remove set on copy using inplace=True

[andywong36]

Currently getting this warning.

foundry/foundry/evaluation/marginal_effects.py

Line 492 in a4bb55e

df_mapping[fname].fillna(midpoints, inplace=True)

/home/__REDACTED__/lib/python3.9/site-packages/foundry/evaluation/marginal_effects.py:492: FutureWarning: A value is trying to be set on a copy of a DataFrame or Series through chained assignment using an inplace method.
The behavior will change in pandas 3.0. This inplace method will never work because the intermediate object on which we are setting values always behaves as a copy.

For example, when doing 'df[col].method(value, inplace=True)', try using 'df.method({col: value}, inplace=True)' or df[col] = df[col].method(value) instead, to perform the operation inplace on the original object.