Ordered execution of Init/Step/Exit/ExitCondition functions

[ptheywood]

Use std::vector instead of std::set for insertion-ordered execution of host functions.

This means that init/step/exit functiond and exit condition functions are executed in the order they are added to the model, rather then the previous sorted by function pointer order.

Closes #707

This may potentially influence the results of models, so is a potentailly breaking change.

[Robadob]

Looks good.

Could be worth updating docs to specify the order thing, I had assumed there was a note about lack of order. But apparently not.

https://docs.flamegpu.com/guide/3-behaviour-definition/6-host-functions.html

[Robadob]

https://github.com/FLAMEGPU/FLAMEGPU2/blob/master/include/flamegpu/model/ModelDescription.h

The doc comments for add init/step/exit all have a @note about order not being guarnteed. This should be updated

[ptheywood]

Could be worth updating docs to specify the order thing, I had assumed there was a note about lack of order. But apparently not.

I'd assumed it was in the docs not docblocks too, and was going to sort after it was merged. Will update the docblocks and open a PR against the userguide too.

[Robadob]

gj

[ptheywood]

Related Docs PR is in: FLAMEGPU/FLAMEGPU2-docs#63.

Should be merged after this has passed CI and been merged.