array-slice interpolation

[stevengj]

Currently array_slice.cpp sets snap_empty_dims=true when calling loop_in_chunks. This means that slice locations are "rounded" to the nearest pixel rather than being interpolated. (See also the discussion in #661.)

It would be nice to do proper interpolation here:

1. In array_slice.cpp, pass snap_empty_dims=false when calling loop_in_chunks

2. Include the interpolation weights in the fields stored in the slice array.

3. Call collapse_empty_dimensions from dft.cpp to collapse empty dimensions to the interpolated result.

[stevengj]

Once this is resolved we can fix the get_array_metadata function, which now has a hard time because DFT slices and array slices are different so the same metadata can't necessarily be correct for both.