This PR adds the standard Aqua.jl tests, introduces a continuous integration workflow (CI.yml) and finally restructures the files in the test folder into a set of reference tests.
Introducing the Aqua tests highlighted a few cases of type piracy; I removed these by introducing small helper functions, rather than extending functionality from LinearAlgebra and Base onto types that this package doesn't own.
There was also a bug in the compat entries for Makie (0.2 instead of 0.20 - not the same thing in semver!), but the bug was only on master, not released.
The CI.yml workflow is untested. I hope it works, but if not, I am happy to tweak it.
I hope the changes will be seen as useful.