Cmake Dependency Download Improvements

[ptheywood]

Following on from #316, there are improvements that can be made to improve quality of life with CMake:

• [ ] Add messages when dependencies are beign downloaded, in case of poor internet to make this obvious to the user.
  • If CMake >= 3.17, then Reporting Checks would be a nice feature to use
• [ ] Allow users to provide their own versions of these dependencies, with CMake options documented on how to provide them
• [ ] Allow users to prevent downloads from happening, using existing verisons or provided version (i.e. if there is no network available)
  • I.e. cmake .. -DUSE_FETCHCONTENT=OFF or similar.
• [ ] Support locally modified versions of dependencies
  • During development of say an external example, it may be neccesary to change the core FLAME GPU 2 library for isntance, but without wanting to push to FLAME GPU 2 itself (as development)
  • Local redirection of the dependency would work?
  • Might be nice to support modification in _deps that then doesn't fail to configure. FETCHCONTENT_UPDATES_DISCONNECTED might be relevant.
• [ ] Ensure VISUALISATION_GIT_VERSION etc can be used via the GUI / for reconfigures. All of these could/should also be applied to the Visualisation repository, although using find_package for some of the vis dependenies was incorrectly finding some things inside conda installs for some user(s)
• [x] Ensure that fetched and found CMake (i.e. GLEW on windows) will only find the fetched version (ideally it shoudl check the host for a matching version first, but Rob's conda install was causing issues iirc)