Modernise Dependent & Included CMake Projects

[ptheywood]

Creating this central issue rather than N smaller issues in each other repository.

Once #661 is merged, other FLAMEGPU2-* repos should have a similar (but much less substantial) improvments made to their CMake to modernise them and enforce the same CMake minimum version.

In general these changes should be relatively minimal.

The Visualiser may benefit from a bit more effort, as it get's effecitvely upstream from the main repository.

Things to consider:

• Do not modify CMAKE_<LANGUAGE>_FLAGS
• Use per target properties / functions. Private where possible, public/interface where required
• Potentially add an aliased FLAMEGPU::flamegpu target? This might need some thought. This may also not be sensible until CMake install project #260 as this might change how it works, and less breaking changes is better?
• Pin against non-master FLAMEGPU2. May impact Example Suite Repository/Repositories #603.

In general, look at the diff of #661 for inspiration.

It may be worth waiting until #653 is figured out too, to reduce effort.

Repos:

• FLAMEGPU/FLAMEGPU2-visualiser
• FLAMEGPU/FLAMEGPU2-example-template
• FLAMEGPU/FLAMEGPU2-tutorial-cudacpp
• FLAMEGPU/FLAMEGPU2-ensemble-benchmark
• FLAMEGPU/FLAMEGPU2-concurrency-benchmark
• FLAMEGPU/FLAMEGPU2-circles-benchmark
• FLAMEGPU/FLAMEGPU2-CINECA
• FLAMEGPU/FLAMEGPU2-python-example-template (probably not required)

[ptheywood]

CMake in the visualiser repo has been modernised to match the main repo previously.

Other repositories contain minimal CMake, so no real modernisation required. The Finding of FLAME GPU will be improved once we have the abillity to install flamegpu #260.

I've updated each standalone repo which uses cmake to pull in the main repository to pin against v2.0.0-alpha.1, as this will allow us to make breaking changes to APIs without having to udpate all standalone repos (if we don't want to maintain all of them at all times).