[cutorch potr*] API parity for potr* functions in cutorch

[killeent]

Addresses #596, #579.

• Potri now expects a call to potrf first. This is consistent with the Torch API: https://github.com/torch/torch7/blob/master/test/test.lua#L2582
• Potr[i|f|s] now accept upper/lower triangular arguments, as supported in torch