Rename Dat properties dataset -> set, ddataset -> dataset
Sparsity is defined on a single pair of DataSets
Implement DataSet.__eq__
Add dset in set and tests for it
Add dat in dset and tests for it
Rename Map.{dim -> arity}
Add DataSet to op2 public API
DataSet needs __{get,set}state__ to be pickle'able
Update pickle load in adv_diff_mpi demo
Rename Map.{dataset -> toset}
Remove set property from Dat