Build Python component as separate sub-package

At the moment, because oxford_asl is built separately for each platform, the Python component is copied into $PREFIX/lib/pythonX.Y, i.e. it is coupled to the Python version that was installed at build time. This means that oxford_asl will need to be rebuilt whenever we update the Python version that is shipped with FSL.

I think the most elegant work-around to this problem may be to build the python and C++ components of oxford_asl as a separate sub-package. It is possible for a single subpackage to be built as a noarch: python package, as outlined here:

https://github.com/conda/conda-build/issues/3702#issuecomment-527764001

There is a problem with this approach, in that we need to build the C++ sub-package on each platform, so the noarch: python sub-package would unnecessarily be getting built several times, and we would need to publish only one of those builds. I haven't yet found any information on how to work around this issue.