feat/bug: Wrap-around of chi coordinate

[pbeaucage]

Problem: slicing chi in cyclic space is somewhat non-trivial - xarray doesn't have built-in support for this, and adding it as a custom coordinate class is problematic.

The immediate solution, which is a bit gross, is to up the priority of #13 and use rsoxs.slice_chi to properly do the necessary (multi-) slicing.

Useful tests:

if array is np180:
slice(270) --> -90
slice(170,190) --> slice(170,180)+slice(-170,-179)

if array is 0360:
slice(-90) --> 270
slice(-10,10) --> slice(350,360)+slice(1,10)

[pbeaucage]

This is now working, with some additional bugfixes lately. Needs documentation.