Make it possible to specify what tests to run.
Add ShellCommand
Add Task
Add Link
Ensure attribute constraints are checked
Add DAG (with minimal validation_errors implemented)
Add DAG name to error
Add error for links to non-existing tasks
Add error for orphaned tasks
Add error for missing manditory inputs
Add error for links to invalid outputs
Add error for when two links target the same input
Add error for cycles of tasks
No need to duplicate role
Add ConvertsToHashref
Add missing use statement
Add Hashref tests
Add to/from_json (for DAGs only)