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.

Use `EXPECT_FEQ()` to ensure robustness against floating point inaccuracies (same thing below).