bug: 2D geometry plots didn't take perspective into account.

[pfebrer]

This PR addresses the issue raised here on discord.

Perspective wasn't taken into account for drawing atoms in 2D and as a result the atoms on top were just the last atoms in the geometry, not the ones that were actually on top.

Now the perspective is automatically determined by your axes choice. I.e.:

import sisl
import sisl.viz

bilayer = sisl.geom.bilayer(top_atoms="C", bottom_atoms=["B", "N"], stacking="AA")

bilayer.plot(axes="xy")

Shows you the view from the top:

and

bilayer.plot(axes="-xy")

Shows you the view from the bottom:

This has been added to the documentation. New tests have also been added.

[codecov]

Codecov Report

Merging #382 (6a4e9fb) into master (a4343f3) will decrease coverage by 12.99%.
The diff coverage is 96.36%.

@@             Coverage Diff             @@
##           master     #382       +/-   ##
===========================================
- Coverage   87.12%   74.13%   -13.00%     
===========================================
  Files         321      321               
  Lines       41895    41933       +38     
===========================================
- Hits        36499    31085     -5414     
- Misses       5396    10848     +5452     

Impacted Files	Coverage Δ
sisl/viz/input_fields/dropdown.py	75.29% <80.00%> (-18.61%)	⬇️
sisl/viz/plots/geometry.py	89.76% <100.00%> (-2.43%)	⬇️
sisl/viz/plots/tests/test_geometry.py	99.24% <100.00%> (-0.76%)	⬇️
sisl/viz/backends/plotly/_plots/pdos.py	0.00% <0.00%> (-100.00%)	⬇️
sisl/viz/backends/matplotlib/_plots/pdos.py	0.00% <0.00%> (-100.00%)	⬇️
sisl/viz/backends/plotly/_plots/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
sisl/viz/backends/plotly/_plots/fatbands.py	0.00% <0.00%> (-100.00%)	⬇️
sisl/viz/backends/plotly/_plots/bond_length.py	0.00% <0.00%> (-100.00%)	⬇️
sisl/viz/backends/matplotlib/_plots/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
sisl/viz/backends/matplotlib/_plots/fatbands.py	0.00% <0.00%> (-100.00%)	⬇️
... and 100 more

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