Updated Goedecker potentials

[mfherbst]

@mkrack recently contributed an extended set of GTH potentials to DFTK (JuliaMolSim/DFTK.jl#950). The new location where the CP2K GTH potentials are stored is https://github.com/cp2k/cp2k-data/tree/master/potentials/Goedecker/cp2k. We should mirror them here or integrate with that repo.

[mkrack]

I suggest just to mirror the relevant sub-folders of the cp2k-data repository in the case of the GTH (HGH) potentials to JuliaMolSim/PseudoLibrary/pseudos/gth. These folders are the feedstock for the CP2K database file GTH_POTENTIALS used by CP2K. A subset of these GTH pseudopotentials has recently been tested in the framework of an exhaustive verification study. I propose to keep the same naming conventions, i.e. to use GTH instead of HGH, to avoid confusion.
The DFTK parsing of path and file names for the pseudos appears to be case insensitive which makes gth/pbe/al-q3 and GTH/PBE/Al-q3 equivalent.