Add PyomoModel class

[khaeru]

This is an incomplete¹ PR that addresses #119 by adding ixmp.model.pyomo.PyomoModel, a class that solves models via Pyomo. Pyomo offers the option of using some supported open-source solvers.

The Pyomo docs use the phrase algebraic modeling language (AML) to refer to an information/data model that includes equations, parameters, variables, etc. Because ixmp was geared towards GAMS, another AML, it already aligns well to these concepts, so the implementation is not too awkward or complex. (The same would apply to other AMLs like AMPL, linopy, and perhaps others.)

¹ it is provided for exploration, discussion, and to clarify the work that would be involved to complete it.

How to review

To be added.

PR checklist

• Complete the code. This is currently blocked by the following limitations of JDBCBackend, that would need to be addressed, either through changes to the underlying Java code (ixmp_source) or a new backend, e.g. Add Python DB API backend #400.
  • Does not allow to set values of equations (ItemType.equ) and variables (ItemType.var). This is necessary to store the model solution produced by Pyomo.
• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[khaeru]

Per discussion with @glatterf42, if work continues with this, it may be better to target linopy, a newer package which seems to be a more performant—though possibly also less mature, uncertain.