Message input/output to same list from multiple functions in same layer.

[Robadob]

This should be possible.

For shared message output; it requires tracking sharing an offset into the message output buffer (and initially sizing it correctly). For shared message input; it requires only triggering buildIndex() once per layer, rather than per agent function.

This would be useful for Primage, where we have multiple cell types all sharing identical messages for cell-cell forces.

Not worth implementing until concurrent functions within layers is operational.

Related: #194 (Throwing exceptions if user attempts to do this)

[ptheywood]

If implemented this may impact the sorting of message lists to improve performance

[ptheywood]

Atleast one user has a use-case for this, where the absence of concurrent message output within a layer is negatively harming performance, due to the overheads / serialisation imposed by using multiple layers instead.

See Discussion #1124