General principles for how to divide schemas between openMINDS modules

[aeidi89]

We need to come up with general principles for how to divide schema between modules, and when this is done, write about these principles in the developers' guide. This is necessary for ensuring consistency over time. It would be good to place schema where they case less dependencies.

Example: the concept schemas Device and DeviceUsage are currently placed in openMINDS_core, while the activity concept schema DevicePlacement is placed in openMINDS_specimenPrep.

This issue is based on a conversation between @lyuba and @apdavison