Draft: Consolidation into a library of basic atomic integration and related routines

Code in arw.f (formerly duplicated in Src and Src/psoplib/src) and related files has been consolidated into a new library in Src/libatom. The relevant module is atomlib_m. Naming might not be optimal.

The criterion for inclusion of routines in libatom/atomlib_m.F is to avoid extra spurious dependencies of dftu_specs on atom. Hence the inclusion of a routine for the generation of the soft-confinement potential, which is not really "basic".

Fixes #308 (closed)