Costchecks are not using GlobalRef

[lkdvos]

Currently, the costcheck implementation uses literal references to TensorOperations methods, instead of GlobalRef, thus failing whenever this is not available in the scope of the macro.

It's an easy fix, just posting this issue here so I don't forget.

[Jutho]

This was also silently included in #171 , so I think this can be closed.

[lkdvos]

If I don't forget, we should probably try to add some tests for this at some point as well