Data validation and schema

• issues

  • bugs #13, #14, #15, #23, #24
  • enhancement #16, #17, #18, #19, #20, #21, #22, #25, #26
  • maintenance agreg_type and join_type

• Validate

  • add new module ntv_validate (class Validator)
  • add Ntv.validate method (type / value validation)

• documentation

  • add ntv-pointer documentation (test_pointer.ipynb)
  • add org. and sch in global Namespace

• Schema

  • Internet-draft for NTV Schema (extending JSON Schema)
  • new module ntv_schema
  • documentation (Jupyter Notebooks)