Schema versioning: tighten validation on fields

[mferrera]

We can and should increase the validation performed on fields in the schema. Some examples:

• the schema version is of the form X.Y.Z where X, Y, Z are positive integers
• URL adapters to give consistency for things like whether or not a URI has a trailing slash (foo.com/ vs foo.com, both valid)
• Define ranges for numerical values to the best of our ability where appropriate (>= 0, > 0, etc.)
• File paths (relative, absolute)
• etc

This will give extra data quality assurance and is best added before a broad roll-out.