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`.