Allow tuning matplotlib subplots.

[pfebrer]

Now subplot kwargs are passed directly to plt.subplots in the matplotlib backend, and one can therefore pass things like gridspec_kw:

import sisl
p = sisl.geom.graphene().plot(axes="xy")
sisl.viz.subplots(p, p, rows=2, gridspec_kw={"height_ratios": [2, 1]}, backend="matplotlib")

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 87.36%. Comparing base (2b299e3) to head (f37c45d).

Files	Patch %	Lines
src/sisl/viz/figure/matplotlib.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #781   +/-   ##
=======================================
  Coverage   87.36%   87.36%           
=======================================
  Files         396      396           
  Lines       50706    50706           
=======================================
  Hits        44298    44298           
  Misses       6408     6408           

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