Create basic visualiser

[nic-barbara]

This is to track progress on development of a basic visualiser - don't merge to main just yet :)

The main idea is to create a visualiser built off the LyceumMuJoCoViz.jl package that is easy to use and integrate into custom simulation/control loops. As per #3, the syntax we want is something like this:

viewer = MuJoCoViewer(model, data)
while <window is open>
    # do controls, data.ctrl = ....
    step!(model, data)
    render!(viewer)
end
close!(viewer)

The render!(viewer) step should be set up with mouse/button callbacks to make the visualisation interactive.

[nic-barbara]

Current status: basic viewer that displays the model, but no interactivity yet. We should try to use the PhysicsState type fromLyceumMuJoCoViz.jl to integrate their default callbacks and handlers into our visualiser. This should be reasonably straightforward once we have nicer array handling on the structs.

[JamieMair]

For this PR, I suggest we can commit the files in vis (except for basic.cc) after some minor tweaks, but we should remove the lyceum-vis from this branch and put it in the next PR. You can ignore all the comments on the lyceum code for now, as it will only be useful as a guide for the next PR.

We should probably move to package extensions in the next PR. It doesn't seem too involved. We can follow this post for a guide, and also the docs. This should allow us to use Requires.jl as well to support people on older versions, before v1.9.

[JamieMair]

alloc is fine here, but we should add a finaliser to GC the objects since this is a mutable struct.

[JamieMair]

Remind me what this C_NULL is again?

[nic-barbara]

Just NULL but in C.

[JamieMair]

I mean the argument to the function, why are we setting it to null? The function calls it pert but I'm not sure what that is.

[nic-barbara]

It's either perturbations to the scene (eg: dragging around the view with the mouse) or perturbations to the environment (eg: due to dragging around objects in the environment). I've set it as null for now because we don't need either of those for the most basic visualiser.

[JamieMair]

This is all great - we should move the frees into the GC.

[nic-barbara]

Good point. How do we go about doing that?

[JamieMair]

We should probably wrap these objects in a type just like Model and Data to make things easier

[nic-barbara]

Agreed. That's happening in #35 right?

[JamieMair]

Does this need the using UnsafeArrays?

Also, good idea to validate the path. Perhaps there is a mj function which also validates an xml file that we can use? But that's for a separate PR.

[nic-barbara]

Possibly not, I'll check.