support OpenFOAM package(s) (#3528)

* ENH: add package for building OpenFOAM (1612) from www.openfoam.com
- provide 'openfoam' as virtual package.
- package as openfoam-com to reflect the distribution point.

This initial spack packaging for OpenFOAM supports a number of possible
variants and should handle 64-bit labels properly now that the scotch
package has been updated accordingly.

* ENH: update package for foam-extend (extend-project.de)

- provide 'openfoam' as virtual package.

- much of the build is now aligned with how the openfoam-com package
  looks, with the aim of future refactoring.

- avoid installing intermediate targets.

- contains its own environment sourcing script for the build, for more
  flexibility and robustness (doesn't touch the python build environ)

* ENH: added package for building from openfoam.org

- provide 'openfoam' as a virtual package.

- this is largely a direct copy of the openfoam-com package.
  It has been supplied as a courtesy for users and to ensure maximum
  consistency in quality and naming between the foam-extend,
  openfoam-com and openfoam-org packages.

* CONFIG: add openfoam into bash completion providers list

* ENH: have openfoam-com use spack as USERMPI

- also simplify the generation of mplib/compiler rules

* ENH: have openfoam-org use spack as SYSTEMMPI

- this setup requires more environment settings than USERMPI
  (openfoam-com), but is currently the only means of integration
  for openfoam-org

- simplify generation of mplib/compiler rules

* ENH: simplify generation of mplib/compiler rules (foam-extend)

- rename mpi rules from SPACK,SPACKMPI to USER,USERMPI for consistency
  with openfoam-com and to generalize for any build system.

* STYLE: record spack tree as a log file (openfoam)

- can be useful for future diagnostics and general record keeping