Improve ellipse related functions

Hello,

The current ellipse functions (like trim/extend) do not work properly, I want to help clean it up.

I have opened my first pull request #106

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Thanks! I will try to clean up more.

While being a qcad user years ago, I noticed that geometry functions for ellipses (and other curves beyond circles) were not properly implemented. I'm glad that I can improve the math part thanks to your LibreCAD setup.

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