Define core CDM vs local extensions

[david-i-berry]

The current CDM defines a core set of tables. We need to revise these and identify what should be mandatory (i.e. present in all implementations) and what is optional. We also need to define / document the process by which local extensions can be defined and linked to the core model. A good example is reanalysis feedback tables - the columns present will vary between models and one table (or family of tables) per model would be more appropriate than the EAV approach.