2021.07 Updated tasks and tests

• All tests have been changed so that to the left of == assert is the correct version of the output, and to the right is the output from the task.
  Now, pytest-clarity shows in green the correct option, in red what is received from the task
• Updated pyneng utility and pytest.ini config files - removed pytest option due to pytest-clarity update
• updated tasks, most of them small changes, clarification of the wording
• removed task (test): 15.2a
• Updated pyneng_common_functions.py file in exercises directory

More about the changes