Add loadable_variables='coords' option

[TomNicholas]

is there a reason one would not want to always include all coordinates in the loadable_variables

The main reason though is that coordinates can be N-dimensional in general, in which case you might use a lot of storage duplicating them. But for low-dimensional coordinates I agree we should recommend people load them.

Perhaps we should even add another option: loadable_variables='coords' or loadable_variables='1d_coords', which we could default to... The latter is pretty much what xr.open_dataset already does for choosing which coordinates to create indexes for.

Originally posted by @TomNicholas in #330

[TomNicholas]

In fact if we do this then why not just change the default for indexes too, or even get rid of it entirely. Loading 1D coordinates and creating indexes, but keeping 2+D coordinates virtual and not trying to create indexes for them (which I don't think is even possible in xarray right now anyway) would be much more intuitive and sane default behaviour, and it would mean all our examples don't need the indexes={} kwarg in them anymore.

In other words we want the default behaviour to be the same as xarray except that data variables and multi-dimensional coordinate variables are virtual. I think I was just over-complicating things.

[TomNicholas]

In #353 I added the options, but making them work with the indexes kwarg for all possible cases is a lot of effort for basically no gain as far as I can tell. I think I will just remove indexes entirely.