Move ForceBalanceBaseClass and BaseReader to __init__

Since ForceBalanceBaseClass and BaseReader are needed by a number of other
forcebalance classes but don't do anything themselves, they can be moved
to init as forcebalance.BaseClass and forcebalance.BaseReader. This is
more in line with standard python programming practices and consolidates
common "global" classes and variables used across the forcebalance package 
into one place.

This also aids in clarity and helps produce self-documenting code since you can
do something like `from forcebalance import BaseReader` or just reference 
`forcebalance.BaseReader`. This is in comparison to the previous forms where
`from forcebalance.basereader import BaseReader` is unnecessarily redundant, and
`from basereader import BaseReader` is less informative to where BaseReader
is coming from

Note also that I renamed ForceBalanceBaseClass to just BaseClass since
forcebalance.ForceBalanceBaseClass seems overly wordy, but if this
causes any issues we could just keep the current naming scheme.