DOC: Document entire range of arithmetic methods in api.rst

doc fixup
CLN: Refactor core/ops + add Panel to it

Cleanup imports in tests/test_expressions
Add numexpr for both series and panel ops
CLN: Disable numexpr for sparse objects
add a test for commutative addition
CLN: Various fixups + disable r* comparisons b/c test failures.

Disable r* comparisons for now because they cause test failures
Move out com fill zeros (I guess evaluate TypeErrors more frequently
Specifically allow radd in TimeOps for Series