Refactored Python (POC) implementation for OpenEO compliance tests

[soxofaan]

I took some to time to get the pytest based compliance test suite working.

• refactored all python related things to a separate directory openeo_compliance_tests
• improved the helper classes so that the actual tests can be more to the point with less boiler plate code
• combined API tests to a single file test_general.py for now because most tests are just 3 lines of code
• added 0.4.0 and 0.4.1 schemas

[soxofaan]

FYI the travis builds are now failing (e.g. https://travis-ci.org/Open-EO/openeo-backend-validator/builds/547730079) because of actual API compliance failures

for example:

• https://earthengine.openeo.org/v0.3/collections/AU/GA/AUSTRALIA_5M_DEM

  jsonschema.exceptions.ValidationError: 'meters' is not of type 'number'
  Failed validating 'type' in schema['properties']['eo:gsd']:

• http://openeo.vgt.vito.be/openeo/0.4.0

  jsonschema.exceptions.ValidationError: 'paid' is a required property
  Failed validating 'required' in schema['properties']['billing']['properties']['plans']['items']:

• http://saocompute.eurac.edu/openEO_0_3_0/openeo/collections/openEO_S2_32632_60m_L2A

  jsonschema.exceptions.ValidationError: 'name' is a required property Failed validating 'required' in schema['properties']['provider']['items']: