Create a service descriptor template generator and validator

[jkerr5]

Generate a service descriptor based on a few inputs:

• Name
• Layer
  • Geometry type
  • Primary data source (view name, SPARQL)
• Others TBD

Also support validation of a descriptor with indication of missing fields or errors in the structure.

[mfgumban]

#66 should be done first before working on this.

On validation:

• validation should be available as a module exported function, making it usable within the core library or be invoked ad hoc in a script or QConsole.
• a custom Gradle task that invokes the function can be provided as a sort of "command line" tool. Gradle tasks project #65
• perhaps JSON schema / Schematron can be used as the validation method.

There could be 2 aspects to validation:

1. Structure: check if the JSON was properly declared and constructed, expected value types, etc.
2. Functional: will the descriptor actually work? are the declared TDEs, indexes, etc. valid?

[jkerr5]

This may be OBE in the future as there are new features in MarkLogic 10 and 11 that make a lot of what is done in the service/layer descriptors easier.