AsFEM build infrastructure

[schruste]

The installation - even when considered together with the installation instructions - is not very comfortable.

I managed to install AsFem after several attempts on my arch-based linux, but I did not like that I had to fix/correct several parts of the installation due to several implicit assumptions used that should be made explicit. A few points:

• In the instructions a debian-based OS is assumed (see the prequisite packages and apt install ... instructions). I think restricting to debian-based systems is OK, but please add sentences that other OS-users will need to adapt...
• Several things are hardcoded in the cmake-build-files. I suggest to use the full power of cmake. cmake has variables that can be used to let cmake search for directories for dependecies (PetSC / MPI / ..). Global environment variables don't need to be used here. Currently global environment variables shall be used to described dependencies. I suggest to a) replace these with cmake-variables and b) to check for the existence of the correspondingly needed libraries in the corresponding location before starting the build process.
• A full setup script could also be provided, e.g. similar to the github-workflow-file.
• if a Docker-Setup is set up, see #62 I would also recommend to publish the full Docker-file as it is also a documentation of the required dependencies and the setup.

[yangbai90]

Dear @schruste , thanks for the suggestion, I have updated the CMakeList file according to your comments.