OpenADR 2.0b VEN client QualityLogic certification

[alexcarnero97]

Certain functions of the VEN client have been modified in order to pass the OpenADR 2.0b certification. The QualityLogic Test Tool has been used to test the library, and the changes introduced are necessary to pass all the mandatory tests. In order to achieve this, we merged the existing Nostromo-energy implementation with ours. Aspects to be taken into account:

• The code has not been formatted using flake8.
• No new tests have been provided as we have used the Test Tool for certification.
• Related issues: Implement one-shot history reports #18 (they have been called "single" reports), ReportSpecifier.report_interval not supported  #123, EiOpt is required for certification #128, VEN has to answer with an oadrCreatedReport to VTN's oadrCreateReport #141, Client should check the ven_id in _on_event #146, (VEN) Return EventResponse/OadrCreatedEvent with specific responseCode #165.
• For simplicity, commits have not been signed.

Our goal is to certify our product and we are happy with the current implementation. However, we would like to contribute to the library with our work.

[axmsoftware]

Merged multiple unsigned commits.