quick fix bugs in stats

[sergpolly]

addressing stats issue #199

the issue stems mostly from the fact that dist_freq became a dictionary by distance-bin instead of being an ndarray (before v1.0) - then there 2 different interpretation of that dict happen:

1. keys being dist_bin themselves in __init__, add_pair, add_pairs_fromdataframe_chunk ...
2. keys being bin_id of a given distance (i.e. bin_id = np.searchsorted(self._dist_bins, some_distance)) in places like from_file and __gfetitem__

This is minimal fix PR - where we switch to dist_freq - as a dict with dist-keys - i.e. changing from_file and __getitem__

Then there is just a simple bug/typo in flatten, where dist is incorrectly defined for the very last open-ended distance interval - it is fixed here as well