Define some general-purpose utility functions for interpreting and selecting regions and computing seasonal climatology
Add region functionality to AMWG plot set 5.

This requires adding region functionality to a couple of the
functions in reductions.py.

Note that region is implemented in the plot set as a class
attribute when the set is initialized, and can be called as
self.region.  Therefore it is not necessary to pass it to methods
in the function as an argument.
Delete set-specific method interpret_region, which is replaced by shared function in reductions.py
Merge pull request #27 from susburrows/susburrows/add_regions_support

Susburrows/add regions support