cmake, Python packaging, and spack with separate build responsibilities

[Helveg]

The responsibilities of our build systems have historically been mangled, with a pseudo-Python-install carried out by CMake as the most problematic and brittle aspect. This PR will fix that as follows:

• CMake carries out the building of the NEST library
  • CMake is by default configured to round out the Python install by calling pip as a pypkg build frontend.
• Python packaging happens (for now) by either carrying out a full CMake build, or a noop build:
  • The full build uses skbuild as a build backend that runs the CMake build without the trailing pip build.
  • The noop build assumes that the binaries were produced by CMake, and just copies them into the output bundle.
• spack will configure a CMake build without pip, followed by a pip build without CMake

This allows us to move away effortlessly from Python, or add new interpreters/bindings without our build tools becoming hypercomplex

[github-actions]

Pull request automatically marked stale!