Resolve "Implement a class for initial conditions from analytic functions"

What does this MR do?

Implements grid function for initial conditions out of a math parser.

Is there something that needs to be double checked?

• Before merging with master, the in-place installation of muparser has to be removed. It will be installed directly on the dune-envdocker image.

Can this MR be accepted?

• Implemented
• !129 (merged) Accepted
• Added test
• Documented (Doxygen)
• Documented (Sphinx)
• Pipeline passing
• Added entry to CHANGELOG.md

Related issues

#131 (closed)

Closes #134 (closed)