- Embed the matrix ``M`` into a space of real matrices.
-
- The matrix ``M`` can have entries in any field at the moment:
- the real numbers, complex numbers, or quaternions. And although
- they are not a field, we can probably support octonions at some
- point, too. This function returns a real matrix that "acts like"
- the original with respect to matrix multiplication; i.e.
-
- real_embed(M*N) = real_embed(M)*real_embed(N)
-