Allow to install both lib mpi and non mpi

[PhMakowski]

Hi, it would be nice to allow installing both version in parallel. Today it is not possible because there are files in common that differ. (headers for example)

[andrea-iob]

At the moment, I don't see how it would be possible to change bitpit to install both MPI and non-MPI versions in the same directory. What I usually do is: I install the two versions in two separate directories and then I set the CMake BITPIT_DIR variable of the applications using bitpit to one of these two directories.