Add `Criteria#<<(criterion)` to stop repetition

Prior to this change `Nexpose::DynamicAssetGroup#criteria` accessor was used to get the DAG's criteria, `Criteria#criteria` was then used to get the actual array following `Array#<<(criterion)` which would be used to add a new criterion. Producing the longer syntax `dag.criteria.criteria << my_new_criterion` vs. `my_dag.criteria << my_new_criterion`