The big one

Main new features:
==============

* More object oriented, with two new classes:
    - `LAPSolver` (for the simple closest positions assignment)
    -  `CMSSolver` for the second close-merge-split linear assignment problem, which kinda work now

* I don't think there were lots of API changes at the `LAPTracker` API level : the `get_track` and `close_merge_split` methods haven't changed.

* Some new features:
    - a `colors` property that assigns a unique color to each segment
    - a  `relabel_fromzero` function (out of the class) to, well, reset the segment labels to zero.


TODO
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Still a bit fuzzy about cost computation, I'll rise issues on the specific points once the PR is passed