Blender Git Loki

Cleanup: Alembic, renamed and moved convert_matrix function

The `convert_matrix()` function just converts between the Alembic and
Blender matrix data types, and doesn't do any coordinate system conversion.
To clarify this, the function has now been renamed to
`convert_matrix_datatype()`.

I also moved the implementations next to each other in the source file,
so that it's visible that there are actually two of them.

No functional changes.