Add Matplotlib magnetization coloring

[Alexboiboi]

Scope

When using the matplotlib library there is no straightforward way to plot color gradients properly in 3D. In this PR, a new set of utilities allows to plot the magnetization orientation over every magnet by slicing and retriangulate the underlying graphical mesh object.

Examples

This is how it looks like

• in qt from a script

- in jupyter notebook

Caveats

So far it is an opt-in behavior, and we need to decide if we make it the new default for the matplotlib backend.
There are a few caveat though that will influence this decision:

• The slicing tends to exacerbate the classical underlying matplotlib issue of bad 3D layering. See example below

• Without deactivating ant-aliasing it is impossible to make the triangle edges disappear. And without antialiasing, the plots looks much uglier in the jupyter notebook, since the dpi is set to 80. If we increase dpi, the plots are getting huge.
  This is how it looks so far:

Antialiasing OFF:

• QT from script:

• jupyter

Antialiasing ON:

• QT from script:

• jupyter

###Notes

• If we implement Set a dynamic default backend depending on the current environment #595 , we won't have to focus so much on the jupyter side of the plots. For now, in the docs I implemented a svg renderer by default, that looks equivalent to the QT output. So that the docs don't look so pixelated.

Open Tasks

• Make it the new default, or not
• Keep default dpi
• Make docs nicer by enforcing svg output (only for the docs)
• @OrtnerMichael feedback

Please @OrtnerMichael try out the feature and help us make the decision on how to move forward ;)

[codecov-commenter]

Codecov Report

Merging #597 (c861a25) into main (0e93067) will decrease coverage by 0.13%.
The diff coverage is 98.27%.

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@@            Coverage Diff             @@
##             main     #597      +/-   ##
==========================================
- Coverage   99.80%   99.68%   -0.13%     
==========================================
  Files          54       54              
  Lines        6218     6296      +78     
==========================================
+ Hits         6206     6276      +70     
- Misses         12       20       +8     

Impacted Files	Coverage Δ
magpylib/_src/fields/field_wrap_BH.py	100.00% <ø> (ø)
magpylib/_src/display/backend_matplotlib.py	97.80% <75.00%> (-2.20%)	⬇️
magpylib/_src/display/traces_base.py	100.00% <100.00%> (ø)
magpylib/_src/display/traces_generic.py	99.00% <100.00%> (-1.00%)	⬇️
magpylib/_src/display/traces_utility.py	100.00% <100.00%> (ø)
magpylib/_src/obj_classes/class_BaseDisplayRepr.py	100.00% <100.00%> (ø)

[OrtnerMichael]

decision:

• coloring default in matplotlib (its crappy no matter what)
• antialiased= True
• keep default dpi

[Alexboiboi]

@OrtnerMichael, ready to merge

[OrtnerMichael]

looks good ty.
we should think about making pyvista or plotly the default backend