Bilayer again

[tfrederiksen]

Some further polishing:

• keep the rotation axis in the middle of the cell (generates visually more intuitive structures in my opinion)
• make layers of identical atom types if only one layer (but either of them) is provided
• added BA stacking (different from AB for stacked heterolayers)

[codecov]

Codecov Report

Merging #186 into master will decrease coverage by 0.00%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
- Coverage   85.19%   85.18%   -0.01%     
==========================================
  Files         120      120              
  Lines       19075    19079       +4     
==========================================
+ Hits        16251    16253       +2     
- Misses       2824     2826       +2     

Impacted Files	Coverage Δ
sisl/geom/bilayer.py	93.54% <80.00%> (-3.01%)	⬇️

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[zerothi]

This is now merged! Thanks!