Coregistration forces users to filter out non-spatial variables

[JanisGailis]

When running coregistration on a dataset that includes non-spatial variables, such as variables only having a time dimensions, a warning message is presented and the user is asked to filter out such data variables.

Expected behavior

Coregistration should skip such datasets instead.

Actual behavior

Describe what Cate does instead or doesn't do at all.

Steps to reproduce the problem

1. Open Ocean Color dataset
2. Try to use it as the dependent dataset in coregistration.

Specifications

cate 2.0.0.dev21