update: MPI version of the code written and tested with direct H*v
The MPI code is available with precompilation flag _MPI

-bug: the MPI code shows a small but possibly important bug: the
eigenvalues obtain in N>1 cpus are NOT machine precision identical
to those obtained with N=1 (i.e. serial) cpu. CHECKING IT.

PLEASE USE THE SERIAL CODE FOR THE TIME BEING.