Pyranges

Changes:
- Make col first argument of assign

Changes:
- Move pyranges db to own module to remove mysql-requirement (made wheelmaking hard)

Additions:
- add assign and subset methods on pyrange

- Only refer to and use ray in dispatcher

Fixes:
- raise Exception when encountering non-"+-" Strand values

Additions:
- pr.sandbox.Debug context manager for pipes

Fixes:
- coverage errored with value_col

Additions:
- Can set columns on a PyRanges using a dict of iterables
- gr() takes subset and col argument, like dplyr mutate and select

Removed:
- disallow eval string, must use lambdas, e.g.: gr(lambda df: df.Score > 0)

Fixes:
- drop (and getitem) small fix
- sometimes had empty dfs in dict because of unused categoricals