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.

Without transposing the matrices, the semantics of the conversion operation are wrong.
Conversions should return a matrix that is equivalent to the source matrix, but in fact, 
we were returning the transpose of it.
Asking clients to remember to always transpose the matrices is inconvenient and error prone.
I'll fix the comments.