Improve interpolation with conda-forge.

[rainwoodman]

It appears that the mpich package on conda-forge is finally compatible with the new anaconda compiling toolchain. We shall rebase our binaries on top of conda-forge.

This will also allow us to get rid of a few other 'platform' packages, e.g. mpi4py, healpy (which has been causing problems). Also means we will be able to pullin their pytorch.

However we do not want to port nbodykit their, since the NERSC scripts still use the recipes here for building. In principle though we can fetch conda-forge recipes and use those at nersc; but that's less urgent.