Converted project to UPM package format

[hybridherbst]

This makes installation, usage, updates much easier.
Follow-up steps could be properly packing the Samples so they can be installed right from PackMan.

[hybridherbst]

@rsodre for visibility.

[rsodre]

hmm, I'm not sure if I want to make that shift now.
I understand that it's easier to install, but this is not a final solution, I always change and tweak things when using it. If this is imported as a package, the user won't be able to do so.
I would not be comfortable working with this as a package myself.

There were zero requests for installing this as a package, I don't want to enforce this. My general conclusion is that Unity is kind of a mess actually, and people already are used to copying and paste things from project to project.

But I can add your fork to the README for those who prefer it.

[hybridherbst]

Haha, I don't agree with "zero" requests, I'm one and I did the changes based on requests from others that wanted to compare various fulldome solutions :)

Note that changing to a package structure doesn't mean that usage gets any different if you want to modify it -

• you can totally clone this repo here as submodule and use the package as a "local package" to develop in it,
• people installing from a registry can "embed" the package and thus it becomes mutable/editable, ...
• the repo can contain "work projects" that reference the package locally and where edits etc. can be made

Let me know if you have other questions about that package workflow.

Would you be interested in some of the fixes that are bundled into this PR as separate PRs? (e.g. d895738 and 607e496)