Moving geometry/multi/ into geometry/

With this small chunk of changes I'd like to start the process of (re)moving the multi directory.
Do we still agree that this should be done?

This first test commit moves the core. It should be simple to move other parts too (maybe besides the algorithms) so I'll probably add them to this PR. But I think it would be better to merge smaller chunks to see if everything is ok on all platforms.
I also tweaked the headers in some files to include only the required functionalities of TypeTraits and Range.

For backward-compatibility the original multi files #include only the non-multi header. In the future we can put some #warnings in there to inform about the deprecation and later remove the directory.