Allow for the use of open source solvers

[dlaehnemann]

The installation of Gurobi is a bit of a pain, especially as it requires separate licenses on different machines and will thus only work on a cluster if you register a license on every single machine. And if you are not working in an academic job, it might even prevent people from using scVILP altogether. Thus, allowing users to use an open source alternative might be a good addition. A number of them can even be installed directly via conda-forge, e.g.:

• conda create -n coin-or-cbc -c conda-forge coin-or-cbc: https://github.com/conda-forge/coin-or-cbc-feedstock
• conda create -n glpk -c conda-forge glpk: https://github.com/conda-forge/glpk-feedstock
  And in Python, you can interface with both of them via PuLP: https://coin-or.github.io/pulp/guides/how_to_configure_solvers.html