Split out 'arborenv' as an installable library from the sup library. (#679)

Make a new installed library `libarborenv.a` covering a subset of the `sup` library functionality, with corresponding installed CMake target `arbor::arborenv`.

* Move NVML or CUDA 10 API decision for GPU UUID discovery to top level CMake.
* Move affinity, concurrency, MPI init guard, and gpu detection and negotiation functionality out of `sup` and into new library `arborenv`.
* Move `include/arbor` in project tree to `arbor/include/arbor` (for consistency across `sup`, `arbor`, and `arborenv` subdirectories.)
* Wrangle more explicit library dependency adding CMake code into the installed `arbor-config.cmake`, to help mitigate [CMake issue #18614](https://gitlab.kitware.com/cmake/cmake/issues/18614).
* Have `arborenv` code throw `std::runtime_exception` instead of `arb::arbor_error`. (We are still using `arb::mpi_error` though for a failure in `with_mpi`.)
* Move `scope_exit` into the `arb::util` namespace.
* Merge `affini...