Feature/lib install target part 4 (#531)

All example code and validation tests no longer require access to private include directories. This provides the minimal requirement for an installable target

Note that it is still not possible to separately build mechanisms from NMODL with just the public includes, and there is not yet any package configuration file creation for use with CMake or pkg-config.

* Replace `hw::node_info` with `proc_allocation`, describing local resources for the purposes of domain decomposition.
* Group processor counting and gpu counting implementation under `node_info.cpp`.
* Remove `domain_decomposition` dependency from `cell_group_factory.hpp` so we can use the latter to test for backend support for a cell kind.
* Add `arb::cell_kind_implementation()` which performs the mapping from cell kind and backend kind to a `cell_group_ptr`-producing function (this will then become the site for custom cell group kind mapping support in future work).
* Move headers for aux library ...