The PEXSI-only processes do not call 'init_spin', and rely on a
broadcast from the Siesta_worker nodes to get certain information. As
nspin, spinor_dim, et al were integer pointers in 'm_init', they were
undefined on entry to the PEXSI interface routines, causing segfaults.
They have been converted to plain integers for now, pending a review
of other possible ways to fix the problem. 

Some pointers in sparse_matrices have been initialized to null().

A test to catch non-collinear or spin-orbit cases has been added
before the call to the PEXSI solver.

Note that the PEXSI routines still use the 'nspin' variable internally
to refer to both 'spinor_dim' and 'h_spin_dim'. This will be changed when
mixed-spin solving is implemented.

Added Tests/sih-pexsi-spin.

modified:
  Src/compute_dm.F
  Src/m_spin.F90
  Src/siesta_analysis.F
  Src/sparse_matrices.F
added:
  Tests/sih-pexsi-spin