Fixed foundation <-> Imath matrix conversions

- A transpose operation was missing when converting to / from Imath matrices. Fixes issue #624
- Replaced individual convert to / from Imath tests with roundtrip tests. 
- Added a test that demostrates the need of adding the transpose operation in conversions.

These methods used to *reinterpret* matrices (i.e. a free operation, by opposition to a copy). Transposition would typically be done on the client's side. I'm fine changing this behavior to make the API easier to use; make sure to update the comments as well.