In #119, we pair controllers together based on the name of the attribute. If the name of two subcontrollers foo_1/foo_2 or Foo1/Foo2 is the same then they're grouped together in the same pvi group under foo.

Problem

This is assuming too much on the FastCS side. We should instead allow controllers to have a ControllerVector called foo which can be indexed by int (not necessarily contiguous). On the transport, these elements are grouped together under foo.

Required Changes

1. Add a new ControllerVector which can be passed into register_subcontroller.
2. Update demo to use this for the ramps
3. Unit tests