cons_tres enhancements

Pass "first_pass" and "avail_cores to _eval_nodes() so that
  the usable cores can be better identified by the GRES selection
  logic.
Add new function, _select_cores(), to select specific cores for use
Create new data structure with job multi-core spec
Permit off-socket cores to be used with enforce_bind
  Needed so that cores on and off socket can be used. Details will need
  to be handled in _select_cores()