When reading model, give error if mandatory JSON attributes are missing

When reading model, give error if mandatory JSON attributes are missing. Mandatory attributes:

• JsonSchema.name (inherited by JsonCustomSchema, JsonJdbcSchema)
• JsonCustomSchema.factory
• JsonMaterialization.sql
• JsonLattice.sql
• JsonTable.name (inherited by JsonCustomTable and JsonView)
• JsonCustomTable.factory
• JsonColumn.name
• JsonFunction.className
• JsonMeasure.agg

Note that JsonFunction.name is not mandatory: a class with several methods defines several functions, each inheriting the name of the method.