Develop Class/Sub-class hierarchy for Activities

[lopierra]

e.g. should certain Data Categories only be permitted to have specific Data Types?

[putmantime]

Currently there is no Assay class and DataFile class is overloaded with properties/relationships that should be on an Assay Class.

We need to model and implement an Assay class and refactor DataFile.

[lopierra]

Changed the subject of this issue to align with current work on Activity class.

[lopierra]

rolling over into March