Allow per-species use of 'user-basis'
(This is just a first version: Please edit or comment below to sharpen the specification)
It might be useful to specify that some species' data can be read from a .ion
file. Currently it is all or nothing.
A possible way could be to allow a list in the user-basis line:
user-basis [ "Si" "H_ghost" ]
Each entry must match a species name in the ChemicalSpecies block.
Note: no support for renaming of .ion files or for "SIESTA_ION_PATH", as this is much more specific than a pseudopotential.