Use options.generate_tests in Generators + various changes

- Apply a check to all generators in Molgenis core:
    if(options.generate_tests) --> generate test classes
    else --> generate classes
- Add entity REST API tests
- Move molgenis generator execution to parent pom
- Enable molgenis test code generation by default
- Enable REST API for all apps (except omicsconnect, see issue #79)
- Cleanup MolgenisOptions molgenis.properties