Susburrows/add regions support

Note that I moved the set-specific method interpret_region from AMWG set 13 to reductions.py, from whence it can be shared by various functions.

Also note that I implemented the region as an attribute in the AMWG plot set 5 class, which is given a value upon initialization.  It is therefore accessible within the class as self.region, and does not need to be passed as an additional argument.