feature request: drop cplex as the only possible solver

[brynpickering]

Would be good to support other solvers other than CPLEX, for a few reasons:

1. The CPLEX python library can't be installed on an Apple Silicon device.
2. CPLEX is a commercial solver. It effectively means OASIS isn't reliably testable or usable by others. Outside academia, you need to pay for a license. Access to CPLEX on academic high performance computing clusters is also not a given.

There are lots of python libraries that allow you to create an optimisation problem that can be solved with a range of solvers. E.g.:

1. Pyomo (I would recommend using their kernel library as it reduces the overhead that Pyomo adds to generating the problem).
2. linopy.