Filter refactor

This is a biggish refactor, so I'd like the blessings of the interested parties before merging. Highlights are

1. Refactor filter-related code out of `fsps.py` into `filters.py`
2. Refactor the filter table itself out of the python code and into a text data file. Now we only need to maintain the short filter names in `fsps/data/filter_keys.txt`. Long filter descriptions are merged in from `$SPS_HOME/data/FILTER_LIST`.
3. Made a script, `scripts/fsps_filter_table.py`, that generates a *nice* restructured text table of the filter set. I'll include this in future revisions to the sphinx documentation. In the meantime we can use this to check that `filter_keys.txt` is consistent with `FILTER_LIST`.