STRUMPACK 6.3.0

• Change default sparsity reducing ordering to use METIS_NodeNDP
  (from METIS_NodeND)
• Significant performance improvements in the GPU code for the
  direct solver, from the NERSC December 2021 Hackathon
• Performance fix in symbolic phase
  (affecting only some MPI implementations)
• Bump minimum CMake version to 3.17
• CMake fix for Perlmutter
• Compilation fix for GCC 8
• Add support for single precision HODLR/Butterfly
  (now requiring ButterflyPACK >= 2.1.0)