Remove option to mark relocating rows with np.nan.
Remove check for NaN flags in MNLLocationChoiceModel

When doing LCM prediction, it will be up to the
user to pass in only the choosers who are picking new
locations. Both RelcationModel and TransitionModel return
indexes of choosers, those can be concatenated to to make
an indexer of choosers.
Merge pull request #23 from synthicity/no-mark

Don't use NaN flags for marking movers