Improve documentation of callback functions

[chrhansk]

This should fix #153 and hopefully generally improve readability of the documentation of the callbacks.

[moorepants]

Thanks for this. I built it but it doesn't seem to work with the numpydoc format:

[moorepants]

I take that back, it does work, but the formatting is a bit unusual with the H1 or H2 in the sub bullet lists.

[moorepants]

Here is SciPy's solve_ivp docstring that also requires a callback function:

That seems sufficient to me and still fits in with the numpydoc format.

May I suggest that at least the "Parameters" and "Returns" headers in the sublist are turned into just bold **Parameters** or not have any formatting applied?

[chrhansk]

That is a good suggestion, I was not entirely happy with the formatting either. I made the changes you suggested, please have a look.

This may be some issue with my setup, but it seems to me that part of the list of attributes is cut off and not visible in the documentation for some reason (i.e., the hessianstructure and intermediate sections). Can you confirm this?

[moorepants]

Thanks. I confirm that part of the docs aren't being rendered:

That's odd.

[moorepants]

Is it somehow because of the multiple blank lines here?

[moorepants]

That fixed it for me. Remove one blank line above **Parameters**.

[moorepants]

I removed the line and pushed a commit.

[chrhansk]

Thanks, appreciate it.