Use Alt/Az when selecting neighboring runs

[IevgenVovk]

Issue: The find_run_neighbours() method uses ICRS coordinate system when selecting the run neighbors, which may lead to runs with very different zenith distances to be stacked when evaluating the background.

Suggestion: use Alt/Az instead of ICRS in find_run_neighbours():

1. introduce az_bins and alt_bins keywords to the configuration file with the following behavior: if integers, they will define the Az/sin(Alt) grid to be used when searching for neighbors; if lists, they will define the Alt and Az grid bin edges;
2. add the alt/az neighboring grid creation in BkgMakerBase.from_config_file();
3. change the pointing_delta argument of find_run_neighbours() to neighboring _grid or similar and provide the grid from step 2 here.

[LukasNickel]

I would suggest to follow the LST behaviour here, which is to match in sin(delta) and cos(zenith) where delta is the angle to the magnetic field. See for example cta-observatory/lst-sim-config#2

The only problem with that is that it then becomes nontrivial to define a distance threshold. We usually only select the n nearest nodes, but with no notion of an absolute distance...
One could of course create a grid and group runs that end up in the same cell