Add tooltips to test parameters on test creation page

[gidili]

From @rgerkin: When creating tests, it isn't obvious what some of the parameters mean. I'd like to have tooltips there. Where could I put these in the corresponding NeuronUnit code? Could they be fields in the schema that SciDash could use to generate tooltips?

[gidili]

This is straightforward if we add tooltips on the NeuronUnit side and the best place would be to keep the tooltips together with the schema as an extra field that then can be propagated to the UI alongside with the other stuff and adding the tooltip itself at that point is trivial.

[rgerkin]

There is an open issue in (scidash/sciunit#84) about formalizing docstrings, and HBP is collaborating with me on figuring this out. The result will be docstrings for every test which can be easily parsed (e.g. with Sphinx) to generate the content of these tooltip.

So I would make this very generic for now, e.g. SciDash calls Test.get_definition(param_name), and I implement that in a simple way for now (e.g. manually for one test), and the tooltip shows if the return value of that method is non-empty. Then later I can implement get_definition correctly to lookup from docstrings, and be able to expect that SciDash will show the new, informative tooltips for all tests with proper docstrings.