Initialization of "ortho" (or "diag") group taken out from mp_startup routine. The reasons for this are
- to avoid unneeded dependencies: currently each and every code that initializes mpi depends upon LAXlib,
  including those that never diagonalize a matrix (e.g., ld1.x). This is completely ridiculous.
- to be free to choose the parallelization of linear algebra after data is read and metabolized.
  It used to be done like that some time ago, by the way.
IMPORTANT NOTICE: all codes that use parallelization of linear algebra MUST explicitly call mp_start_diag.
Currently this is done by: cp.x, pw.x, neb.x, ph.x, projwfc.x. All other codes cannot currently use linear
algebra parallelization (linear algebra will run on 1 processor).