enable GEMMT with openblas, mkl

[minrk]

adds build variant with openblas, with GEMMT enabled. As a result, installing mumps with openblas should get GEMMT by default. Installing mumps with any other blas should still get the most-compatible build targeting netlib.

closes #120

Draft because I'm still doing some performance testing to see if this is worth doing.

[conda-forge-webservices]

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/meta.yaml) and found it was in an excellent condition.

[minrk]

On my laptop (10 core M1 macbook), GEMMT does not appear to be worth it, at least for a simple poisson solve:

OMP clearly gives a benefit in single-process solves (much less so in multiprocess, as expected), but at least openblas GEMMT doesn't seem to affect performance much, and when it does, the effect is not always positive.

[minrk]

Apparently my test didn't actually exercise the GEMMT path, but when I try cholesky route that actually uses GEMMT, it's way slower, in fact. Not sure what's up with that. Trying with MKL as well.

[conda-forge-webservices]

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

I wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found some lint.

Here's what I've got...

For recipe/meta.yaml:

• Failed to even lint the recipe, probably because of a conda-smithy bug 😢. This likely indicates a problem in your meta.yaml, though. To get a traceback to help figure out what's going on, install conda-smithy and run conda smithy recipe-lint . from the recipe directory.

[conda-forge-webservices]

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/meta.yaml) and found it was in an excellent condition.