Make PIT's data field searchable

[bertspaan]

Make PIT's data field searchable with Elasticsearch

Steps:

• JSON schema for a dataset's data field, in dataset.json
• Only support unnested values:
  • strings
  • booleans
  • numbers
  • dates
  • (or arrays of those types)
• New module which creates Elasticsearch mappings from data field's JSON schema: data-schema-to-es-mapping
• Check data field in IO
• Define new queries in API!

Affected repositories:

• histograph/data
• histograph/io
• histograph/core
• histograph/api