DOC: New example Biot-Savart for line sources

[SylvesterOester]

Biot savart compared to bipole.

[coveralls]

Pull Request Test Coverage Report for Build 3259353294

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.679%

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Totals
Change from base Build 3256797105:	0.0%
Covered Lines:	3108
Relevant Lines:	3118

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💛 - Coveralls

[prisae]

Hi @SylvesterOester, thanks for your contribution, that is great!

I updated the formatting to (a) make it compatible with sphinx-gallery, and (b) bring it a bit inline with other examples (e.g., fullspace instead of whole space).

It would be great if you could have a look and give your approval, you can find the rendered version here:
https://empymod--155.org.readthedocs.build/en/155/gallery/fdomain/sil_biot_savart.html

Now, there is one thing that should have some more information: Biot-Savart, as you give it, is frequency-independent, is that right? And for empymod, you use f=83 Hz. How does that affect your result? How would it work for, say, f=1e-5 Hz or f=1e5 Hz? Does it always hold? How can that be? It would be great if you could add an additional paragraph about that (also to your thesis by the way.)

[prisae]

OK, I made some tests. As long as the frequency is low enough (say, below a few 10k Hz), there is not much change in this case. So while 83 Hz might look arbitrary, it doesn't really matter.