Molecule and cross-reference blocks

[jgaff]

Decided after discussion:

• molecule should be a top-level block, that contains an object (not a list)
  • vendor will remain
  • chemical_formula will be removed - material.composition is the canonical field for formulas
• material will remain unchanged
• alloy will be stubbed out as an example of a non-molecule material/substance
• composite will be stubbed out as an example of a multi-material record
  • composite will be defined as an object containing arrays of molecule, material, etc. objects

[jgaff]

To stub out alloy and composite, the schemas will be created in the prototypes directory and not linked to in the record schema, meaning they cannot be used.