add plotting utilities

[landinjm]

This should not be in the JuliaMPBSolver package as it adds bulk that we don't care about for the python wrapper.

[j-fu]

Right. If we nevertheless want to do this, we could do this in a package extension which would be loaded only when a plotting package is loaded.
See e.g. https://discourse.julialang.org/t/quick-tutorial-on-package-extensions/130923