test fixes to ExprWithLimits and children/related

- change usage of .subs() in risch.py
- fixes a test bug caused by improved scoping of ExprWithLimits.subs()
- integrals.free_symbols now calls inherited form
- ExprWithLimits free symbol change, treat undetermined forms consistently
- ExprWithIntLimits into a new file, roll in functions from simplification.py
- remove subs test docstring failures for Integral
- adding docstrings to ExprWithLimits
- cleaning up code