Fix value trying to be set on a copy of a slice from a DataFrame from tests

[brews]

From a warning at the bottom of the running tests.

tests/test_rff.py::test_process_rff_sample
  /opt/hostedtoolcache/Python/3.10.12/x64/lib/python3.10/site-packages/dscim/utils/rff.py:211: SettingWithCopyWarning: 
  A value is trying to be set on a copy of a slice from a DataFrame
  
  See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
    rff_df.weight[np.isnan(rff_df.weight)] = np.exp(

Prob want this so code isn't setting a value on a copy of a slice from a DataFrame, if this is the case.