Find a way to avoid customizing map_coordinates

[astrofrog]

We have a wrapper of map_coordinates in array_utils.py which pads the array before being called - however this causes the input array to be loaded into memory, whereas if we didn't do this map_coordinates from scipy could deal well with memory-mapped arrays.

We should explore this again and see if we can avoid having to include this wrapper.

[Cadair]

Also presumably dropping the wrapper would help with the future use of any dask-image variant of map_coordinates?

[astrofrog]

Indeed!

[astrofrog]

We're going to have to use the wrapper for the foreseeable future as it also now handles the fact that the default map_coordinates loads the whole array for big-endian data. However, we no longer have memory issues in ours, so we can close this now.