add test to check parent-child resource relationship. Add structure to many to many tests
init structure for many-to-many fields research
fix link presence from parent to child model
update foreign key check. Require multiple relationship from the other side.
init many to many test, hold results
update test for many to many relationship
add implementation for many to many fields
unify method (make similar to method with the same functionality)
add tests to resource many to many from other model definition
add many to many investigation from other related resource
add model many-to-many investigation from related's model child test
respect pylama