select static samples

[antgonza]

Figure out a way to select a group of samples that should always be used for the random selection of samples, an easy way can be by metadata. This feature will be useful for temporal or any other gradient studies, as you could set that always selected group as those from time 0, then the controllers will only allow you to select parameters between those samples and you can run your calculations.

[wdwvt1]

This was the source of an error that occurred recently where we needed to have certain samples that were time0 points selected to compare them to other time points. Resolving this will be critical for effective use of the compare_categories.py functionality.