Remove Plots from REQUIRE

[schillic]

The library should not have the requirement Plots.jl, which is only used for plotting. We have RecipesBase.jl for defining the plot recipes.

[mforets]

does removing this requirement conflict with having plot commands in the unit tests?

[schillic]

We have to add it manually before running the tests/building the docs of course. But to use the package, it should not be required.

[mforets]

But to use the package, it should not be required.

I am neutral on this one, because the user would need Plots in most use cases. But it's true that visualization is is as optional as Polyhedra for concrete polyhedra so i think it is consistent to make the user install it separately.