Subsetting may fail when applied to a dataset with only one value in lat or lon dimension

[TonioF]

When you create a dataset for a certain region and then create a dataset of a sub-region, you would expect to get a dataset.
Instead, you may get an error 'Can not select a region outside dataset boundaries.'
The reason is that in the current implementation of subset_spatial, the resolution of a dimension is determined as the distance between two values of the dimension. If the dimension only has one value, the resolution is assumed to be 0 (whereas it really might be up to 60°), and if the user-defined subset region does not coincidentally contan the actual dimension value, the resulting dimension will have no values, causing the error stated above.