Begin switching to integer indices only

It seems using what I think are called record arrays don't support
numerical operations.
Simplify the dtype

Everything seems like it would fit in a 16 bit signed integer, so use
that for all season summary stats. This greatly simplifies the type of
the array and I think will allow me to better utilize numpy. Maybe.
Redo indexing stats dict

Using string keys would be nice. Except it seems like that's a dumb idea
(shock!). This involves more indirection, but summarizing stats
shouldn't happen very often. At least once it's final.