Add example that showcases quadratic objective / constraints

[rluce]

Maybe start from https://www.gurobi.com/documentation/9.5/examples/portfolio_py.html#subsubsection:portfolio.py and see if things get nicer when using the accessors.

[simonbowly]

This doesn't really illustrate the accessor API too well, as the model constructs only single constraints, not indexed ones.

There is possibly an opportunity for a nice scenario mapping API though.

[simonbowly]

We need another example here, this one doesn't show building of row-wise constraints, the aggregation just gives single constraints.