Add cmake build system

[iamashwin99]

• Add CMake build system
• Make Ninja as the build backend
• add support for mpi variant in cmake

TODO: ( in other MRs)

• add support for more variant
• overhaul the ci: move the build and testing commands into a bash file for modularity

[fangohr]

So this runs through with Octopus 14.1 and cmake and the spack develop version? That's already pretty impressive!

[iamashwin99]

waiting for #112

[lang-m]

Parts of the code should be split off into separate classes as documented in https://spack.readthedocs.io/en/latest/packaging_guide.html#multiple-build-systems