Add the gsXBraid module

[hverhelst]

Adding Multi-grid Reduced in Time (MGRIT) via Xbraid.

This PR is a new version of #471 (by @mmoelle1 @roeltielen) using the updated module system.

More info on XBraid in G+Smo can be found in

Tielen, R., Möller, M., & Vuik, C. (2022). Combining p-multigrid and Multigrid Reduction in Time methods to obtain a scalable solver for Isogeometric Analysis. SN Applied Sciences, 4(6), 163.

Tielen, R. P. W. M. (2021). P-multigrid methods for Isogeometric analysis.

---

NEW:
Add the gsXBraid module, allowing to use Xbraid in G+Smo.

IMPROVED:

FIXED:

API:

---

For each new/improved/fixed/api change use a new line with the
respective word prefix in capital.

Please consider the following checklist before issuing a pull
request:

• Have you added an explanation of what your changes do and why
  you'd like us to include them?
• Have you documented any new codes using Doxygen comments?
• Have you written new tests or examples for your changes?

---