Outsource package-requirement messages to a central place

[schillic]

We have quite a few of these guys, sometimes with slightly different text:

@assert isdefined(@__MODULE__, :Polyhedra) "this function requires the package 'Polyhedra'"

I propose to outsource them to a central function to make the code simpler.
Maybe we should even write a macro, e.g., @required (@require is already taken by Requires).

[mforets]

Yes, a macro @requires_polyhedra, or @requires(:Polyhedra) that displays a message similar to "this function needs the package 'Polyhedra'; do 'using Polyhedra'" and then try again would centralize those messages in one place.

[schillic]

@requires_polyhedra

I prefer to have a generic macro, as in your second proposal. Polyhedra is not the only optional package.
Actually, this makes me realize that we generally want to support a list of packages 👍