restore ability to find METIS

fixes #125

metis 5.2.1.3

example: don't require MPI_CXX

ci: programmatic set FC for macOS

FindLAPACK: correct package name

mumps 5.3: fix source bug

cmake >= 3.20 tested in CI

upstream MUMPS 5.7.3

correct mumps include dir that can cause C++ include failure on macOS

this is due to case-insensitive filesystem snagging the file
VERSION in the top-level of mumps_SOURCE_DIR.

correct scalapack git commit hash