Schemas should be checked against merged dictionaries instead of individual JSON files

[chrisgorgo]

See https://github.com/INCF/bids-validator/pull/301#issuecomment-337293516 for context.

BIDS allows key/value pairs shared among many different JSON files to be stored in single location higher in the directory tree. This is called the Inheritance Principle and is described in section 3.5 of the spec (http://bids.neuroimaging.io/bids_spec1.0.2.pdf).

Currently validator relies on JSON schemas to check validity of JSON files. However, those schemas are checked against individual files instead of merged dictionaries across the hierarchy. This needs to be fixed.

Schemas are validated here: https://github.com/INCF/bids-validator/blob/master/validators/json.js#L34.

It's worth mentioning that merging JSON files across hierarchies is already implemented: https://github.com/INCF/bids-validator/blob/37924a136d9831256bd0eb47606a9b2031cc523e/validators/nii.js#L39

[chrisgorgo]

It's worth noting that right now each JSON file is checked individually and the schemas do not include information about which fields are required. Those checks are done manually in code after the dictionaries are merged.