Developmental stage

Create a field for developmental stage - this probably lives within specific types of subjects.
We can use this for species where ages/date of birth are unavailable as well as for embryonic ages.
This might be worth using everywhere (maybe optionally) - e.g. we know the date of birth for our mice because we breed them. But someone working with mice in different context (e.g. field experiment?) might not have that. So maybe developmental stage is an optional field for all subjects (at the top level).
This needs some leg work to make these decisions, as well as to explore if there are general or species specific ontologies we can use.