Don't use NaN flags for marking movers

When doing LCM prediction, it will be up to the user to pass in only the choosers who are picking new locations. Both `RelocationModel` and `TransitionModel` return indexes of choosers, those can be concatenated to to make an indexer of choosers (using `urbansim.models.util.concat_indexes` (which might move)).

@janowicz This might affect some of your recent work?