Fix infinite recursion when Summing a Piecewise.

Fixes github issue #2597.
Add test for github issue #2597.
return None for piecewise if not small definite
Merge pull request #9 from smichr/piece

return None for piecewise if not small definite
Merge pull request #2659 from jrioux/piecewise

Fix infinite recursion when Summing a Piecewise.