Install simulation methods from pypi

[mberz]

The current method of installing simulation methods by including them as submodules may be a bit error prone when references are accidentally updated. I think it makes sense to replace this by installing (specific versions) from PyPI when possible.

[mberz]

We could also install from git repositories via pip if no release on PyPI is available.

[mberz]

Submodules are removed in

• Refactor/de #33 for acousticDE
• DG-FEM Backend: Replace install from submodule by install from git repo #32 for edg-acoustics

[mberz]

example module install is replaced in #34

[mberz]

Implemented #76