Make trackpy behave like customized_trackpy in mr.

[danielballan]

• Make iterative_link (i.e., new name for link_iterator) the fundamental loop.
• Support link with Tom's original trackpy API.
• Support track (dataframe_link?) with Dan's mr API.

[danielballan]

This is what I settled on:

• At the lowest level, we have link_iter (following Nathan's naming) which returns a generator that yields DummyTracks (Nathan called them TrackNoStore).
• link_df operates on a DataFrame like mr.link demonstrated in my tutorial. I don't love this name -- open to suggestions.
• link maintains Tom's API, wrapping link_iter.

[danielballan]

Closed by #13