Using `data_input: {from_: fitinputcontext}` should be used with great care if at all

Created by: wilsonmr

I need to add to the docs that you really should avoiding this if possible. It's used in a very specific case in the compare fits runcards to bridge backwards compatibility but if it's used liberally it basically overwrites the data_input key in each element of group_dataset_inputs_by* and so each group ends up with every single dataset and your computer will grind to a halt

I'm not sure if running fitinputcontext:: <action involving groups> would do the same thing, I should test this.