Direct support for UPF format pseudopotentials

[prckent]

UPF format pseudopotentials, as used by QE, can include the semi-local potential needed for QMC as well as the non-local Kleinman-Bylander projectors used by plane wave codes such as QE. A big simplification in our workflows would therefore be to support these files directly since users would only need one pseudopotential file. While the UPF documentation is a little ambiguous, the modular reader code used by pw.x correctly skips semi-local components while the pseudopotential generator code has example source for formatting the semi-local data. As far as I can tell, there are no existing potentials with both components.

Since UPF files with projectors are currently being generated for https://pseudopotentiallibrary.org/ , I suggest we plan to use UPF files with both components in QMCPACK as a simplifying step. Modifying QMCPACK to read this data is straightforward. In the same way that pseudopotential generating code information is currently included in the opening comments in the UPF files, I suggest we add (require) specification of the local potential and cutoffs there.