Check variable constraints when accessing external data sources

[wwqrd]

Check external data sources match property constraints on access.

Currently this will only affect manually constructed protocols (Architect has an issue to check assets on import). In future it will be necessary for dynamic imports (those that use URLs).

• Properties to respect the constraint:
  /^[a-zA-Z0-9._\\-:]*$/
• All node/edge/ego variable names
• All categorical/ordinal options values

Relates to: https://github.com/codaco/Architect/issues/645

[jthrilly]

Required before #847 can be implemented.