Subset spatial expands 1D variables to 3D

[JanisGailis]

When spatial_subset is run on a dataset that includes a non-spatial variable with only a 'time' dimension, such dataset will be 'expanded' to a time/lat/lon dataset covering the whole subset.

Expected behavior

subset_spatial shouldn't change non-spatial data variables.

Steps to reproduce the problem

1. Open ocean color dataset
2. run spatial_subset on it
3. Observe the crs variable changing dtype from float to int and having time/lat/lon dimensions afterwards.

Specifications

cate 2.0.0.dev21