Feature Calculator: add documentation in config dialog

[imagejan]

It can be difficult to find how certain features are calculated if you're not familiar with the source code and the project organisation (knip, imagej-ops, etc.).

It would be great to have some documentation for the feature sets immediately available in the configuration dialog of the Feature Calculator node, similar to the way it is done for the aggregation methods in the GroupBy node, e.g. by adding a Description tab:

[tibuch]

@imagejan I agree. I will redesign the UI of the Feature Calculator and make sure that such a description is available. Thank you for pointing it out :)