Allow counting of records to work for series as well as dataframes

abundance_in_group was failing when the lack of an abundance
column resulted in trying to use .count() on a Series instead of
a Dataframe. This replaces that call with .size() which works on
both.

It also removes an unnecessary slicing to remove columns that
should never exist.

Fixes #10.
Merge pull request #12 from ethanwhite/master

Allow counting of records to work for series as well as dataframes