Merge linear_solver_cpr
Dear @d.voskov, @apalha and @luisaforozco,
The main reason for this MR is that we are planning to add HYPRE as a submodule to open-darts that has been already done in this MR. Moreover, it would be nice to not leave the developments of @apalha in a separate branch and keep them in development.
@d.voskov, please check and approve!
I appreciate if @apalha or @luisaforozco could briefly write what has been done within this branch in linear solvers. And perhaps what/how you suggest to finish this work in future MRs. Or please just post a link if you have already done it somewhere else.