Provide an easy way to use nain4

[jacg]

Implement whatever cmake incantations are necessary for nain4 to be discoverable by cmake so that it can be used easily by clients.

NB: this repository contains

1. nain4

2. a Nix flake for installing Geant4.
   The latter is not needed to use the former: nain4 needs some installation of Geant4, but not necessarily this one. People who already have Geant4 installed, will want to continue to use whatever installation they already had. The flake is merely a convenience for

3. Testing nain4 on CI, etc.

4. People who haven't installed G4 yet.

[jacg]

This is mostly covered by #10 and #11, but there are still issues with Catch2 on MacOS (and maybe elsewhere), as discussed in #15.

[jacg]

I propose we close this, and use #15 to track the remaining issue we're aware of .