Rationale: they are a simple wrapper around perform(), which is in the interface of gof.op.

Fixed bugs in omega.grad.Grad to make it deal properly with mixed defined and undefined values.
Added many more test cases to omega.grad.

Changed the rule for upgrading grad() return values to lists, making them more strict.
This cought bugs in pow_scalar functions, which were fixed.
Added exp(), log() to omega.core

added more error checking to gemm, changed names from iadd to add_inplace and similar for other inplace ops.