Validation Updates

[fschrader1992]

This PR

• adds a prototype for property values validation
• uses the list given in formats to check for mandatory attributes for docs, sections and properties during validation
• adds the possibility to validate standalone sections and properties
• adds validation for sections and properties on initialization
• adds respective tests

[coveralls]

Coverage increased (+0.2%) to 75.442% when pulling 329588a on fschrader1992:master into 5cdbce4 on G-Node:master.

[achilleas-k]

Some tests with invalid objects that check if the correct errors are produced should also be added.

More useful would be to test the new functionality. This PR introduces validation on object creation, so the new functionality should be tested: Are the appropriate error messages printed when an object with issues is created? Since we just print the errors to stdout when this happens and the errors aren't stored anywhere after that, it might be necessary to capture stdout in the tests and check if the expected output was printed.

[fschrader1992]

Checks for Code Quality:

• Pylint
• Rebase from GNODE Master
• Repeat Local Tests

[mpsonntag]

Looks good, thanks! One additional small change and one comment (WARNING->ERROR) has not been resolved yet.

[fschrader1992]

Checks for Code Quality:

• Pylint
• Rebase from GNODE Master
• Repeat Local Tests Python 3.8
• Repeat Local Tests Python 2.7

[achilleas-k]

Approved from me, though I'd like to see a few more tests for the validation. Will open an issue as follow-up to this.