Add MPI awareness to LightKrylov

[Simkern]

The current version of LightKrylov is serial, which is not a big problem as most of the heavy lifting computationally is done by the call to matvec, the implementation of which is external to LightKrylov. Nevertheless, if one wants to have information messages at runtime to check what is happening within LightKrylov, it is useful to make the toolbox MPI aware such that these messages can be written by one rank only to not clutter the logfile.

[Simkern]

The present simple implementation makes LightKrylov aware of parallelism.

• LightKrylov_Constants provides MPI initialisation and finalisation routines
• The rank, iorank and communicator size are private variables in LightKrylov_Constants, namely nid, nio and comm_size, respectively. They are accessible via getter/setter routines only.
• The user can choose to use any rank for I/O via nio.
• The implementation is based on mpi_f08 which must be present and linked as an external module in fpm.toml. From what I see, there is no 'serial' version of the library in the sense that it would be natively compatible with serial compilation. Therefore, I have added preprocessor macros that disable the MPI related code and imports in serial cases. The variables nid, com_size and nio are then set to the default value of 1.