Refactor PowExp to error about unimplemented ^^ in the backend.
Add a trait to indicate if a function is override.
Add unit test for traits isOverrideFunction.
Merge pull request #2366 from jacob-carlborg/traits_isOverrideFunction

Add a trait to indicate if a function is override
Merge pull request #2358 from ibuclaw/powexp

Refactor PowExp to error about unimplemented ^^ in the backend.