Rename MPI{Matrix,View} to Xchg{Matrix,View}.

This will avoid confusion when we start using them to exchange
data with the host rather than just other MPI ranks.
Unify some of the packing kernels between backends.
Merge branch 'feature/packing' into develop.
Merge remote-tracking branch 'upstream/develop' into develop.
Include the clBLAS status code when raising.
Cleanups.
Cleanups.
Add support for surface anti-aliasing.  Cleanups to follow.
Cleanups.
Whitespace.
Remove an unnecessary assert.
Allow templates to take a default set of render arguments.
Whitespace.
Merge pull request #32 from FreddieWitherden/develop

Towards v0.2.4.