Spatial subset not aligned with "elements" and "edges" variables

[massimilianofronza]

The "elements" contain indexes referred to longitude and latitude values, and account on these 2 to find the triangle vertices. When subsetting longitude and latitude this indexing gets lost, since the elements are still bounded to the original positions.

Possible solutions:

1. Get a longitude and latitude range like [:N], so that every index from 0 to N will stay in the same place even after the subsetting. Pros&Cons: fast to implement, but you can't select a specific region now(for a pilot data GNN testing it should still be ok).
2. Put the subset indexes in a dictionary along with the lon and lat coordinates, so that the original indexing is preserved. Pros&Cons: more time-consuming to implement and more data to store, with the advantage of being able to select a specific region(again, this should not be a fundamental feature for the proof of concept).
3. Keep the original longitude and latitude variables and just cut the edges. In this way you can reduce the heaviest variable(the edges) while keeping the ordering of indexes.
4. (optional) Search in the notebooks if the guys at AWI had a similar problem when subsetting lon and lat.