Fix Link::external_input and Link::external_output
Set::Scalar should be ->is_empty(), not ->empty()
Test DAG::_validate_link_operation_consistency()
DAG::_validate_mandatory_inputs() error formatting
Fix DAG::connect_input() and DAG::connect_output()
DAG::_encode_target (for mandatory_inputs)
Sort errors in DAG
DAG::_validate_link_targets_are_unique naming
Use Data::Dump::pp in DAG errors
Test DAG::_validate_mandatory_inputs
Test DAG::_validate_link_targets_are_unique