Compute and plot 2D ring geometry

[swhite2401]

This is based on the code proposed by @simoneliuzzo in #433.
Two functions are provided:

• ring.get_geometry() to compute the ring geometry
• ring.plot_geometry() to plot and compute the ring geometry

[simoneliuzzo]

Dear @swhite2401,

I think there is some problem with the radius computation.

When asking for centered=True for a ring lattice, the output radius is zero, and thus the coordinates are unchanged. The resulting figure is not centered on zero.

data, radius, ax = ring.plot_geometry(centered=True)

When computing the coordinates for a subset of the lattice, the radius seems also not correctly computed:

resulting figure is not centered on zero.

data, radius, ax = arc.plot_geometry(centered=True)

I expected the radius to be independent of the portion of lattice requested (unless it is just a drift).

[swhite2401]

@simoneliuzzo, @lfarv, I think this version fixes the issues you have found.
I also remove the input arguments offset as I did not see much added value w.r.t start_coordinates (shifting origin)

The main difference is that now centered=True, will always center the results on (0,0) while in the previous version the offset was applied after re-centering. If you think this is a useful feature I can put it back.

[lfarv]

It seems ok now !