reproject loads whole cube into memory

[keflavich]

This was noted in #506 and is noted in the docs, but we closed #506 without fixing this.

We should have the ability to reproject plane-by-plane without loading the whole cube into memory.

It's not obvious to me that dask can help with this at all - @astrofrog any ideas? Otherwise, we'll use the memmap machinery built for other reasons.

I'm going to need this soon so I'll prioritize it.

[astrofrog]

I think dask should be able to do this in the same way that we do e.g. convolution plane-by-plane? Note also that this is relevant: astropy/reproject#214 - but I haven't had any time to look into it.

[keflavich]

#845 is my starting point