Restructure main data storage to reflect trajectory structure in xarray.

[pirmink]

Is a trajectory composed by a number of segments with all same number of points and the condition does want to ignore when moving between segments?
If so, I would probably solve the problem differently by using a different data structure, a sequence of sequences for lon. So that the algorithms applied to only the inner most sequence can not mixed accross contiguous segments. That eliminates the need of this extra condition, and makes the intention more readable.
But I am not sure I understood it

Originally posted by @cosunae in #18 (comment)