Avro schema for FHIR DSTU3

Initial avro schema for DSTU3 FHIR and example how to load the schema, validate the resources, and store parsed FHIR jsons into avro format.

Implemented resources:

Generic:
    * Batch
    * Coding
    * Period
    * Policy
    * Position
    * Quantity
    * Range
    * Address
    * Attachment
    * Codeableconcept
    * Contactpoint
    * HumanName
    * Ratio
    * Repeat
    * Communication
    * Idetifier
    * Reference
    * SampleData
    * Timing
    * Udi
    * Actor
    * Administration
    * Annotation
    * AvailableTime
    * Collection
    * Communication
    * DateCriterion
    * DoseAndRate
    * FocalDevice
    * NotAvailable
    * Nutrient
    * PackageConcent
    * Processing
    * Protocol
    * ReactionRef
    * ReferenceRange
    * Supplement
    * Texture
    * Component
    * Contact
    * Container
    * Data
    * Diagnosis
    * DispenseRequest
    * Dosage
    * EnteralFormula
    * Evidence
    * Except
    * Explanation
    * Hospitalization
    * Link
    * OralDiet
    * Package
    * Participant
    * Performer
    * PractitionerRef
    * Qualification
    * QuestionaireAnswerOption
    * QuestionaireEnableWhen
    * QuestionaireInitial
    * QuestionaireItem
    * QuestionaireAnswer
    * Reaction
    * Recommendation
    * Related
    * Requester
    * Stage
    * StatusHistory
    * Substition
    * Target
    * UsageContext
    * VaccinationProtocol
    
FHIR Resources:
    * AllergyIntolerance
    * Appointment
    * Bundle
    * Condition
    * Consent
    * Coverate
    * Device
    * DeviceRequest
    * DeviceUsesStatement
    * DiagnosticReport
    * Encounter
    * Flag
    * Goal
    * Immunization
    * ImmunizationRecommendation
    * Ingredient
    * Location
    * Medication
    * MedicationAdministration
    * MedicationDispense
    * MedicationStatement
    * NutritionOrder
    * Observation
    * Organization
    * Patient
    * Practitioner
    * PractitionerRole
    * Procedure
    * ProcedureRequest
    * Questionnaire
    * QuestionnaireResponse
    * Specimen* Questionnaire

Run example:

virtualenv --python=python3 venv
. venv/bin/activate
pip install -r requirements.txt
python process_fhir_to_avro.py