Further migration of VecPhys to use VecCore

This merge request unifies a few more function implementations that had a specialization for Vc, and mostly removes the dependency on Vc, which now allows the VectorBackend defined in the main header to be backend::Scalar from VecCore.

This is part of the work necessary to be able to compile VecPhys with any backend later, and is a part of a second phase of migration to VecCore which will allow compilation against any backend in the future.

The third and final phase in the migration will be to remove all template specializations for the scalar backend, and make VecPhys make exclusive usage of VecCore interfaces.