Add support for "local" par_loops

If we pass the flag only_local=True to a par_loop it will only compute
over owned set entities (even in the case of indirect loops).  Once the
computation has finished, Dats accessed via INC perform a halo exchange
to /gather/ remote increments into their local data.

Such loops only make sense if none of the Args are Mats and if Dats are
only ever accessed as either INC or READ, errors are raised in other
cases.