Improvements in preprocessing

[fmaussion]

This PR implies several major improvements on several preprocessing steps. I'll make more detailed comments before merging, this is still a WIP.

[fmaussion]

Here a non-comprehensive list of the changes involved:

• the downstream lines and their integration with glacier divides is now completely different. (this is the major change of this PR). This was necessary to be able to cope with very complex divide geometries like ice caps. Now the downstream lines can go in all directions, in which case they meet their tributary outside of the domain (see plot below)
• the parabolic bedshape computed from the real topography is now included to the model chain (finally). Again in the example below, one can see that the glacier width adapts to the surrounding topography
• the initialize_flowline task now filters for small and negative slopes. This was introduced by Small slopes filtering now happens at the flowline determination #232 but is now used and tested by default
• The inversion takes rectangular beds into account. this happens when the flowline grid point normals touch a: (i) a neighbor glacier (ii) a neighbor divide (iii) a neighbor flowline catchment
• The init_present_time tasks now takes all this into account (this was the major bunch of work - quite boring but necessary). The default is now to produce a MixedFlowline in all cases, which is a mix of parabolic, rectangular and trapezoidal shapes.
• Tidewater glaciers have rectangular beds at the tongue
• The flowline model can now cope with tidewater glaciers and compute the calving flux (very basic)
• The model flowlines are better tested. Their constructors and documentation are still quite bad though
• I added some examples of random glacier evolution in the getting started notebook

I will do a little bit of cleaning a a few tests before merging.