Add code quality checks

[noyainrain]

Include a linter and/or style checker into the developer toolchain. The goal is to enhance code quality and ease code reviews by:

• checking for violations of coding/style conventions
• checking for avoidable programming errors and bad practices

Add a check.py script with a return code of 0 if the unit tests and linting succeed, 1 otherwise.

[noyainrain]

• Pylint
• Pyflakes - no style checks
• pep8 - only style checks

=> Pylint

[noyainrain]

Also include regular expression checks, that can be applied to all text files.

[noyainrain]

For now, checks are only applied to a subset of the project. To complete this issue, include the whole code base.