add __repr__ methods to all classes in base.py

http://stackoverflow.com/questions/1436703/difference-between-str-and-repr-in-python/2626364#2626364
for discussion of __repr__ functions. Next step is to add __str__
functions to print out contents of objects. The goal here is to create
small logs describing the contents of objects instead of encoding that
in the output dir names in e.g. runEnsemble. Then we can run arbitrary
galaxy models without cobbling together long path names for each
change in PSF, vObsErr, etc.
Merge branch 'master' of https://github.com/mrgeorge/speclens