Replace String Literals with static/const/constexpr

[ptheywood]

String literals for reserved variable names such as _stepCount, _id, _agent_birth etc. are bad.

RTC/Jitify means there is a lot of forward declaration in headers, rather than includes, so we need some sensible way of having constants without having to effectively re declare them.

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From #719

Rather than use raw string literals all over, we should try to use cosntexpr's instead, such as the exisitng ModelData::DEFAULT_STATE inplace of "default".

This is used in some places, but not all.

The same will apply for other string literals such as _id etc.

We could do this for "x"/"y"/"z", but this might not be as desirable.

[Robadob]

"_agent_birth" is used in CUDAAgentModel and CUDAAgent. Should be static const "agent_birth_flag" or similar, owned by CUDAAgentModel. Isn't used on device, it's hashed and combined with agent function name before it gets to device.

[ptheywood]

Ownership within CUDAAgentModel is not idea (although logical) for all cases, as others are used in placed where CUDAAgentModel is (and should due to rtc) only forward declared.

[Robadob]

It works for this case, only other class "_agent_birth" is used in already depends on CUDAAgentModel. Special header may only be needed for stuff shared across host/device.