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.

Using `operator()` is probably suboptimal (unless the loops are fully unrolled, in which case the optimizer can probably get rid of all the multiplications). I would recommend writing the matrix copy/transposition without loops and using linear indices instead.