Update gmm

[volkm]

The failures in #707 might be fixed when updating gmm to a newer version.

I tried updating to gmm-5.4.4, but encountered the same issue as here. Apparently, now either SuperLU or MUMPS are needed. Maybe it is possible to change it such that gmm still works without either of the libraries.

I also tried using gmm-5.4.2 where the issue is not yet present, but this version requires at least some adaptions in GmmxxMultiplier, for example mult_add_by_row_bwd was removed.

When updating gmm we should also revise its handling in CMake:

• download gmm instead of shipping it
• install it as described here which only exports the include files and not the rest.

[tquatmann]

Thanks for looking into this!

Apparently, we did make some custom changes to gmm, see e.g. here.

I believe many (probably all) of these changes are no longer needed since the introduction of the VIOperator. In particular, I think we can get rid of the GmmxxMultiplier. I will look into this.

[volkm]

I looked a bit further and using gmm-5.4.2 should be possible after integrating the changes of #721. However, the issue still persists with this version.
I also tried to use gmm-5.4.4 (commenting out parts which needed MUMPS), but the issue of #707 is still present here as well.

[tquatmann]

We can also take the opportunity to make gmm an optional dependency (that still should be installed by default). Once #721 is merged, gmm++ is only used as a linear equation solver, for which we have alternatives, e.g., the Native equation solver or Eigen.

[sjunges]

Did we report the error to the GMM++ discussion forum?

[volkm]

Yes