Feature request: Polygon aperture element

[roman-martin]

For collimation studies, machines like FCC-hh or SIS100 need a more general aperture element that is not limited to ellipse, rectangle or rectellipse. A list of x-y-coordinates defining a polygon should cover most use cases.

As an proposal, I have written a LimitPolygon class that can be copied directly into pysixtrack.

[roman-martin]

As I needed this anyway, I have implemented it in a separate package for now, together with an adapted MAD-X loader and tests.