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.
Set: maintain sizes as a single tuple

Rather than having a different slot for each of core, non_core, exec and
non-exec Set sizes, just use the tuple we were passed in and index
appropriately in the public properties.
Introduce new LocalSet type

A LocalSet wraps a Set and indicates that iteration should only occur
over owned set elements.  A par_loop iterating over such a set carries
out a reverse halo exchange at the end of computation to ensure that all
INC Args are up to date.
Allow par_loops on LocalSets to have WRITE Args

The user guarantees that the write from either "side" would have been
identical anyway, so we can assume that the local write was complete and
correct, and there's no need to force a halo exchange.
Support LocalSets built on ExtrudedSets
Cache LocalSets on Set
Merge pull request #391 from firedrakeproject/local-par_loop

* local-par_loop:
  Cache LocalSets on Set
  Support LocalSets built on ExtrudedSets
  Allow par_loops on LocalSets to have WRITE Args
  Introduce new LocalSet type
  Set: maintain sizes as a single tuple
  Add support for "local" par_loops