Fix Dependencies

[sandreza]

Right now (on a fresh install), plotting won't happen. See https://pkgdocs.julialang.org/v1/creating-packages/#Test-specific-dependencies-in-Julia-1.2-and-above

I think this means that GLMakie would need to be a weakdep / extension. See the conditional loading of packages section

[sandreza]

Here's another example: https://github.com/pebeto/julia_extensions_example/blob/main/Project.toml