Add option to keep track of centerline indices when cutting them and measuring lengths

[erikmannerfelt]

To measure length differences, the mean length of one year can be subtracted to another. When measuring the spread, however, it is better to do an element-wise subtraction (each element being a buffered centerline's length) and deriving the spread of that. This requires some way of keeping track of the index of each centerline. As far as I'm aware, the order of LineStrings in the MultiLineString from buffer_centerlines() is kept when cutting, but this is not a guarantee unless there's a check for it, and #12 would mess with the order.

One potential solution would be to allow a dictionary as the centerlines in cut_centerlines(). For example:

>>> centerlines = {0: LineString(...), 1: LineString(...)}
>>> glacier_lengths.cut_centerlines(centerlines, cut_geometry)
{1: LineString(...)}

Here, I assume that #12 is implemented, and that centerline "0" was deemed invalid in some way, such as that it didn't intersect the cut-geometry.