Clean up Pandas warnings in eval code

[mdekstrand]

Warning during tests:

/Users/mde48/LensKit/lkpy/lenskit/lenskit/crossfold.py:282: DeprecationWarning: DataFrameGroupBy.apply operated on the grouping columns. This behavior is deprecated, and in a future version of pandas the grouping columns will be excluded from the operation. Either pass `include_groups=False` to exclude the groupings or explicitly select the grouping columns after groupby to silence this warning.
    test = ugf.apply(method)

I expect fixing this may fall out of #371.