make sure Grid sorting is transparent

[dpanici]

Grid will sort the nodes by zeta, which when iota is negative and you are passing in coordinates along a field line, can actually flip the order of the nodes you've just passed in, resulting in the new nodes having a decreasing theta instead of an increasing theta, for example.

We should make this sorting clear/throw a warning when this happens to make it more transparent

[f0uriest]

The confusion mostly occurs when creating a custom Grid, and in that case I think we can just have sort default to false. The only reason we sort things is so they're aligned correctly for doing an FFT, but with a custom grid its very unlikely that the other requirements for an fft would be met (ie equal spacing, same rho/theta at each zeta) so it's probably not worth it for that case.