Adds Filter Node, refactoring and Postman collection

[diegostruk]

• Adds Filter Node.
• Adds validation to ReadCsvNode: throws an exception if the ReadCsvNode has predecessors.
• Adds FIXED_INPUT_NODES and MAX_INPUT_NODES constants to NodeUtils to clean up if/elif in validate_predecessor_data method.
• Adds a Postman collection to run using the test runner: currently checks for response codes and some response bodies. To be able to run it successfully you will need the file sample.csv (I can send you the file through slack) or adapt the calls and nodes created to a file of your choice.