Measuring mu' and mu'' of ferrite cores

[rjordans]

This change adds a new plot type "S11 mu" to show the complex permeability of an inductor core.

Cores are measured by putting one or more windings on them and measuring the S11.

In order to correctly calculate the permeability the user also needs to set the dimensions of the core (cross-section and effective length) and the number of windings. This can be done by opening the context menu for the plot (right click on the graph area).

The core dimensions can either be measured from the core directly using calipers.

More info on how the calculations work are available in #206

A brief description of the new measurement method is also added to the main project README.

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: fixes #206

What is the new behavior?

• The Charts/RI.py was split into a generic complex number plotting chart and a specialized one for real-imaginary impedance.
• Next, a new variant for plotting the complex permeability is added
• Finally, the README is updated to include some usage instructions

Does this introduce a breaking change?

• Yes
• No

Other information

[rjordans]

Also added fix for #589