Add parallelised iDGFM support

[danieludick]

The MPI/OpenMP version of the DGFM implementation should now be expanded with the iDGFM also (i.e. where we need to look at parallelising the calculation of the primary and secondary MBFs used in the Jacobi iterations).

(But when planning the implementation, one should also keep in mind that this bit of code might be useful for the CBFM solver too).