Support Piecewise linear expression

[FabianHofmann]

See
https://www.gurobi.com/documentation/9.5/refman/objectives.html#subsubsection:PiecewiseObj
and
https://www.gurobi.com/documentation/9.5/refman/py_model_agc_pwl.html

[brynpickering]

See also #437

[brynpickering]

@FabianHofmann could you assign me to this as a paper trail for when I find time for it?