remove allophone column before grouping to avoid pandas warning

[MiniXC]

Pointed out in #9

python3.8/site-packages/phones/__init__.py:219: FutureWarning: The default value of numeric_only 
in DataFrameGroupBy.mean is deprecated. In a future version, numeric_only will default to False. 
Either specify numeric_only or select only columns which should be valid for the function.
  self.data.groupby(