DataSet.reset_index() fails using level argument with MultiIndex

[JBGreisman]

DataSet.reset_index() raises a KeyError when using the level argument to specify only a few labels in a MultiIndex. This occurs because reset_index() assumes that all labels are being removed from the index when trying to reassign cached MTZ dtypes:

dataset = rs.read_mtz("tests/data/algorithms/HEWL_unmerged.mtz")
print(dataset.index.names)      # prints ['H', 'K', 'L']
dataset.reset_index(level=['H', 'K'])

Outputs:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~/rs/lib/python3.7/site-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   2888             try:
-> 2889                 return self._engine.get_loc(casted_key)
   2890             except KeyError as err:

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 'L'

The above exception was the direct cause of the following exception:

KeyError                                  Traceback (most recent call last)
<ipython-input-12-93c78908ac77> in <module>
----> 1 dataset.reset_index(level=['H', 'K'])

~/reciprocalspaceship/reciprocalspaceship/dataset.py in reset_index(self, **kwargs)
    135                 for key in newdf._cache_index_dtypes.keys():
    136                     dtype = newdf._cache_index_dtypes[key]
--> 137                     newdf[key] = newdf[key].astype(dtype)
    138                 newdf._cache_index_dtypes = {}
    139             return newdf

~/rs/lib/python3.7/site-packages/pandas/core/frame.py in __getitem__(self, key)
   2897             if self.columns.nlevels > 1:
   2898                 return self._getitem_multilevel(key)
-> 2899             indexer = self.columns.get_loc(key)
   2900             if is_integer(indexer):
   2901                 indexer = [indexer]

~/rs/lib/python3.7/site-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   2889                 return self._engine.get_loc(casted_key)
   2890             except KeyError as err:
-> 2891                 raise KeyError(key) from err
   2892 
   2893         if tolerance is not None:

KeyError: 'L'

Since pandas supports a level= argument to reset_index(), the overloaded method should be modified to only try to change dtypes of columns that are removed from the index.