If it's not a circle nor ellipse nor straight line
convert the curve to BSpline. Move into a subfunction.
Improve a few varible names to be more verbose, like `vertex`
and `edge`.
The angle between the curve and plane is tested for being coplanar,
0 or 180 degrees. The negative test is done earlier so that the
return is done earlier, and the rest of the code is not indented.
For the try-except blocks we should trap a specific `Exception`,
not just any type. It seems string handling was an issue
with Python 2, so the string needed UTF8 decoding.
When the software no longer supports Python 2, these
blocks can be removed.
Small fixes where the `get_svg` function is used, for example,
in the (obsolete) `DrawingView` class and `Arch_SectionPlane`.
Also update the unit tests accordingly.
