C elegans subject metadata

We are trying to help a group get C. elegans data into NWB and DANDI and they are coming up against some incompatibilities in the subject metadata.

From a conversation with @rly and @dysprague:

> I was wondering if there were possible changes we could make to the restrictions imposed on age and sex. C. elegans growth stages are defined from L1-L4 followed by adulthood. Most of the worms we use are designated 'YA' for young adult. I believe for our purposes, this is a more useful designation than dahs or weeks old since the lifesplan of the worm is relatively short. Furthermore, for sex, C. elegans have two sexes: male 'XO' and hermaphrodite 'XX' which are designated by their sex chromosomes. I see that there is an option for other, but since the hermaphrodite is the typical system we study, I think it would be useful to be able to just input the chromosomal designations if that's possible.

1. `Subject.sex` is currently limited to "M", "F", "U", or "O". For C. elegans, would it be possible to also accept "XO" (male) and "XX" (hermaphrodite)? Would it be possible to accept such data in the NWB schema?

2. `Subject.age`. For C. elegans, they are not sure if they can recover the age of the worm. It would be more informative to store the growth stage. We could extend `Subject` to a new ndtype `CElegansSubject` that has an additional field `growth_stage`, and make the necessary changes in NWB Inspector. Would it be possible to accept such data in the NWB schema?

We have started a Draft PR to make these changes for (1) in the NWB Inspector: https://github.com/NeurodataWithoutBorders/nwbinspector/pull/353