Change BlockRanges to class RangePartitioning

Change BlockRanges to class RangePartitioning

Now RangePartitioning is a proper class, apart from the temporary
method that allows direct access to the (private) index.
Range based for loops are now used where possible.
Also simplified some use of this in DD distribute.

Change-Id: Ic7b1895024a74a79a4838253545329bd4c8f27c0