Remaining features for Region

[ZedThree]

We still need to do the following to complete the implementation of Region:

• [x] Implement some method of offsets
• [x] Replace all uses of DataIterator with Region, deprecate DataIterator
• [ ] Replace all uses of RangeIterator with Region, deprecate RangeIterator
• [ ] Replace all uses of Mesh::iterateBndry* methods with Region, deprecate those methods (see #950 for list)
• [ ] Add a RGN_DEFAULT that can be changed at runtime by the user, and use this everywhere (at least where it makes sense for there to be a choice)

[ZedThree]

We also need to think about what regions should be in a mesh by default

[ZedThree]

We can't deprecate RangeIterator until we remove Mesh::iterateBndry*, which I've been struggling with due to the lack of tests on the bits that still use those routines.