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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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