Rebase on top of original assimp sources to keep PyAssimp's commit history intact

[SaladDais]

Now that I think of it, it's preferable to keep the commit history for assimp even if most of it's unrelated to PyAssimp. Make a commit deleting all assimp sources and moving PyAssimp to the repo root, then rebase impasse on top of that.

Rewriting history isn't nice since I've already based my sources on top of a squashed commit, but impact should be mininal since nobody's using this yet.

[SaladDais]

Done