Link variables to methods (and back)

[ravwojdyla]

Follow up from #276.

#276 comes with doc links from methods to sgkit.variables.

We would like:

• to have links between variables and methods both ways
• the links available both in doc and runtime

Some approaches to explore:

• use inspect/reflection to look at the signature and docstring as a heuristic to find links
• add a decorator for "sgkit computation" methods that would require a set of variable_spec inputs and variable_spec outputs, note that it would not require the actual names of them just the specs. Having that it would be straightforward to generate links. Such decorator could also validate some best practices like: method returning a Dataset, merge parameter, keyword parameters etc