Add utility function to efficiently unstack xarrays

[takluyver]

unstack_regular() aims to solve 3 pain points of DataArray.unstack():

1. The data is copied to a new array, even when it could make a view, which is slow and needs extra memory.
2. Integer data can be unexpectedly converted to floats to allow NaN as a fill value.
3. The unstacked dimensions are placed at the end of the axis order, instead of where the original stacked dimension was.

Points 1 & 3 may be addressed in xarray at some point.