minion_rs / minion C++: Allow the use of minion to solve optimisation problems

[niklasdewally]

Motivation

Minion supports solving optimisation problems as well as finding exact solutions. In the Minion input language this is represented as follows:

OptimisationFn::= MAXIMISING <varname> or <varlist>
                             | MINIMISING <varname> or <varlist>

See the Minion manual section A.73 input search for more details.

This issue

Support for these optimisation functions needs to be added to:

• libminion
• minion_rs
• The minion solver adaptor inside conjure_oxide.

Support for optimisation functions in general may also need to be added to the solver interface and/or conjure_oxide as a whole - at time of writing, there is no way to tell the adaptor to optimise not find exact solutions.