Feature/214 additional unit tests node class

This MR provides a new unit test for class Node, called test_node.py. It reaches 100% code coverage, and provides 23 tests.

Possible issues:

• it would be great if reviewer could advise on if it is a good idea to patch validators (in test test_node_14_parser_task_error_found). Currently it is not done
• the test test_node_16_task_manager_normal_case_scenario has to be implemeted (we don't provide any way to stop gracefully the infinite loop in the task_manager method).
• patches in setUp method have been done without decorators