Legend grouping in bands plots

[pfebrer]

This PR solves two problems:

• Legend grouping of fatbands in matplotlib. Since the "fatband" of each band is drawn as a separate line, the matplotlib backend used to show multiple legend items. Now this is solved by only showing one item:

• The bands where shown individually on the legend, which often made for a very useless legend. Now all bands are shown as one item in the legend by default. This can be tuned with the group_legend (in bands plots) and bands_group_legend (in fatbands plots) arguments.

import sisl

# First, we create the geometry
BN = sisl.geom.graphene(atoms=["B", "N"])

# Create a hamiltonian with different on-site terms
H = sisl.Hamiltonian(BN)

H[0, 0] = 2
H[1, 1] = -2

H[0, 1] = -2.7
H[1, 0] = -2.7

H[0, 1, (-1, 0)] = -2.7
H[0, 1, (0, -1)] = -2.7
H[1, 0, (1, 0)] = -2.7
H[1, 0, (0, 1)] = -2.7

band = sisl.BandStructure(
    H,
    [[0.0, 0.0], [2.0 / 3, 1.0 / 3], [1.0 / 2, 1.0 / 2], [1.0, 1.0]],
    301,
    [r"Gamma", "K", "M", r"Gamma"],
)

fatbands = band.plot.fatbands(backend="matplotlib")
fatbands.split_groups(name="$species")
fatbands.axes.legend()

Reverting legend grouping for bands:

fatbands.update_inputs(bands_group_legend=False).legend()

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 69.04762% with 13 lines in your changes missing coverage. Please review.

Project coverage is 87.35%. Comparing base (1d3f4da) to head (d3a6b90).

Files	Patch %	Lines
src/sisl/viz/processors/bands.py	41.66%	7 Missing :warning:
src/sisl/viz/plots/bands.py	85.00%	3 Missing :warning:
src/sisl/viz/figure/matplotlib.py	50.00%	2 Missing :warning:
src/sisl/viz/plotters/xarray.py	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #784 +/- ## ======================================= Coverage 87.34% 87.35% ======================================= Files 396 396 Lines 50750 50757 +7 ======================================= + Hits 44330 44340 +10 + Misses 6420 6417 -3 ```

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