FluxTubes: Additonal approximation

[AHaumer]

When investigating #4740, trying to implement a new material, I found it really hard to calculate correct parameters for the used approximation according to T.Roschke. I have implemented a backwards compatible solution to utilize additional approximation formulae. For now, I've implemented a (slightly modified) formula suggested by W.Macfadyen, and created two parameter records (M330-50A and M400-50A), as well as two records with raw data (magnetization characteristic from manufacturer) to document the source of the approximation parameters. If I manage to get more manufacturers' datasheet, I would implement more parameter records.
Additionally, I added 3 examples demonstrating the usage of the new approximation function.,
and some new pages in the User's Guide explaining the two approximation functions
and the workflow to fit parameters to given characteristics.
Feedback is highly welcome.

[AHaumer]

Additionally, with the RawData.records and a CombiTable1Ds it is possible (in BaseClasses.FixedShape) to implement a table-based interpolation instead of the approximation formulae.

[AHaumer]

Converted to draft. I'll have to apply changes to the RawData record to make it fool-proof.

[AHaumer]

@dietmarw there are not really a lot of electrical engineers available, so it's a pity that you decline the review.

[dietmarw]

@AHaumer I declined it since I don't like to be bombarded with draft PRs messages that are not even ready for review. In future, please do not assign any reviewers UNTIL you are finished on your side.

[AHaumer]

@dietmarw sorry I forgot to change the status to draft after I had discovered the first bugs,
Anyhow, thanks for your valuable feedback. I'll try to avoid asking you for a review or advice.