More helpful exports

[rdeits]

I've noticed that it's kind of annoying to have to do using MeshCatMechanisms just to be able to construct a GUI(mechanism, URDFVisuals(urdf)). We could instead have RigidBodySim.Visualization re-export a few things, since using RigidBodySim.Visualization is a bit more obvious than explaining (and remembering) to load a whole different package as well.

Or we could just have RigidBodySim itself export URDFVisuals.

[tkoolen]

Yeah, I've gone back and forth on this myself. Actually, StaticArrays is in a similar position now with LinearAlgebra, see also JuliaArrays/StaticArrays.jl#464 for some discussion. I'm not sure what the right thing to do is here. One option could be to make it so that running the quick start notebook is possible with only using RigidBodySim.