support for 2D spot detection with TrackPy

[ttung]

Description

#1808 erroneously removed the code that detected whether we are working with 2d or 3d data. however, it is possible to call trackpy spot detection with is_volume=False and get 2d slices. that will bomb the existing code.

It is not a simple matter of reverting #1808, because even without #1808, the code will bomb later on in the constructor of PerImageSliceSpotResults, where the absence of z data will trigger an assertion.

[ttung]

It's quite likely that local_max_peak_finder has some bugs too!

[ttung]

ok local_max_peak_finder has a different set of bugs which I will document elsewhere.

EDIT: in #1814 and #1815

[ttung]

Update: we should make trackpy work in 2D.