Check for assert statements in Arrange block

[jamescooke]

Ensure that assertions are not carried out in the Arrange block - a smell that arrangement is too complicated.

• Add bad examples.

• Update documentation with new error code: "Problematic code", "Correct code", "Rationale", "Exceptions"

• Update CHANGELOG

• Extra: Add example of assert and self.assertEqual() in Act with statement body. This pins this responsibility on the context manager analysis that happens when the Act block is built.