Enable MPI matrix

[duncanmmacleod]

This PR implements a build matrix to build against the different MPI implementations, with the logic cribbed from the HDF5 feedstock.

I have no idea whether mpich is actually a firm requirement, so this proposed change may not be appropriate.

Checklist

• [x] Used a fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [x] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe:

• License is not an SPDX identifier (or a custom LicenseRef) nor an SPDX license expression.

[duncanmmacleod]

I don't understand why the build is failing for openmpi. The azure builds are complaining about conflicts, which I cannot reproduce locally. pymultinest seems to be involved, which makes no sense, since that is only required for the tests.