[Scenario] Visit collections when assembling costs

When aggregating cost functions from children,
consider collections. Since activities from
collections are considered when constructing a
timeline, costs from collections must also be
considered in order to ensure that cost graphs
(and other future views involving costs) remain
consistent. Failure to do so results in
nasa/MCT-Plugins#85
Merge pull request #91 from VWoeltjen/mctPlugins85

[Scenario] Visit collections when assembling costs