ENH: handle restart files with new ADIOS

- ADIOS-2.4.0 and earlier had timeName.bp files
  and timeName.bp.dir/ directories. Restart checked for
  times based on the timeName.bp files

- ADIOS-2.5.0 and later simply use timeName/ directories,
  for which we can use slight modifications of standard
  OpenFOAM fileOperation routines.

Does not handle mixed naming. If the .bp files exist, we assume that
they provide full coverage for restart.