Updated all unit tests correspondingly to the previous commits.

tests/test_leiden_oefenopgave.py

tests/test_leiden_oefenopgave_v12.py

tests/test_node.py

tests/test_parser.py

tests/test_possibilities.py

tests/test_rules_derivatives.py

tests/test_rules_factors.py

tests/test_rules_lineq.py

tests/test_rules_logarithmic.py

tests/test_rules_numerics.py