Rename "order" to "scheme"

[cdeil]

Currently we have order : { ‘nested’ | ‘ring’ }.

In HEALPix, "order" is also often used for k = log2(nside), so IMO this is confusing and should be changed.

I propose to replace by "scheme".

Using "ordering" which is used in FITS serialisation might be an alternative, but IMO "scheme" is better, leaving no room for confusion.

I'm happy to make a PR to adapt the few affiliated packages using this already, but given that this is alpha software, I'd suggest to just do this change and ship it with v0.3 without deprecation / supporting both for a period of time.

@astrofrog and all - thoughts?