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.
Update the version number for v0.2.3.
Update the AUTHORS file
Merge branch 'release/v0.2.3' into develop.
Merge https://github.com/vincentlab/PyFR into develop
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.
Merge branch 'develop' of https://github.com/FreddieWitherden/PyFR into develop
clean up roem