Add initial support for CASTEP / ONETEP file format

Pseudopotential without NLCC support is added.

The file format is given in q space so Fourrier Bessel transformations are applied to store it in real space. This transform is of course not bijective so data is lost in the process. The dump of the pseudo is then done via Fourrier-Bessel transforms to go back to q space. This may require some discussion to know if it's wise to introduce storage in q space also and accessors for both space.