Merged PEXSI-solver interface code
Please see the manual for the new options related to the PEXSI solver. Keep in mind that this solver is competitive only for very large calculations. Alberto Garcia thanks Lin Lin, Chao Yang, Georg Huhs, and Nick Papior for their help with the design and coding of the interface. Developer note: Please see the "Notes for PEXSI operation" in Src/siesta.F for a brief overview of the basic coding changes that need to be kept into account for further development work.
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