Image grid: softer subset verification

[markotoplak]

Issue

Conditions for valid subsets are needlessly strong. I noticed this when fixing something else.

Description of changes

Subset computation could be based on ids alone (as elsewhere), or, optionally here, on image names. The active subset is a union of ids and matching image names.

Still warns the user if an element from subset can not be found in the data.

Includes

• [X] Code changes
• [X] Tests
• [ ] Documentation

[codecov[bot]]

Codecov Report

Merging #222 (bebf955) into master (f78d96c) will increase coverage by 0.15%. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #222 +/- ## ========================================== + Coverage 68.91% 69.07% +0.15% ========================================== Files 17 17 Lines 2815 2826 +11 Branches 392 392 ========================================== + Hits 1940 1952 +12 + Misses 766 765 -1 Partials 109 109 ```

[markotoplak]

We discussed this with @janezd in the morning. Could either you, @janezd, or perhaps @PrimozGodec look at this?

[markotoplak]

@janezd, thanks, looks good except for that small inefficiency. :)