Refactor templates to use class inheritance

[anngvu]

This is currently being tested in the Gray Foundation model first, but it is a nice modeling feature akin to having dry enums. It would allow neater, more understandable templates because properties (slots) would be automatically expanded according to the class lineage, so e.g. scRNASeqTemplate would inherit sequencing relevant props from SequencingTemplate, and the scRNAseqTemplate should only contain props that are specific to single-cell sequencing.