Add instruction on how to compile on cluster with muparser

[oguevremont]

Description of the problem

The new parsing class prevented compilation on the Digital Alliance clusters.

Description of the solution

An instruction on which module to load to allow compilation is added.

How Has This Been Tested?

Compilation of deal.II and Lethe on the cluster works, and all tests pass.

Documentation

• [doc/source/installation/digital_alliance.rst] The required module is added.

[blaisb]

Thanks man, good job finding a nice fix for this