Update CG sum simplifications for the new framework
Update CG sum simplifications for the new framework

In addition to reorganize the CG core simplification functions into the
new framework, the SymPy CG simplification is removed.  It appears that
it capability is extremely limited, which may make it not worthy of the
CPU time spend on it.

tschijnmo/drudge,tschijnmo/drudge,tschijnmo/drudge