added VASPsol++ support, and support for using VASPsol++ with VTSTTools.

[bpachev]

This PR adds support for using the VASPsol++ extension to VASP with version 6.3.2. Support is enabled for VASPsol++ both with and without VTSTtools. Patch files were supplied directly by Craig Plaisance, the author of VASPsol++. Slight modifications were made to the original patch files for compatibility with the Spack install process.

[Dingyusong1229]

Thank you for providing these patches and for clarifying their provenance. Could you please confirm whether vaspsol++-vasp_6.3.2.patch and vaspsol++-vtst-vasp_6.3.2.patch are the original upstream patch files supplied by Craig Plaisance, or Spack-specific modified versions derived from those original files?
Since the description mentions that slight modifications were made for compatibility with the Spack installation process, could you also indicate exactly what was changed? If possible, could you provide the original patch files, their checksums, or a diff against the original versions? This would help users distinguish the official upstream VASPsol++ 6.3.2 patches from the Spack-adapted versions when performing a manual installation.
It would also be helpful to confirm whether the VTST patch specifically targets vtstcode-209. Thank you!