Encapsulate DSL from internal API

[s1ck]

trafficstars

The internal API mainly contains the operator interfaces, the graph representations and the call methods. The current convenience methods at LogicalGraph and GraphCollection reflect the DSL visible to the user. Internal API and DSL are not encapsulated. One idea is to add a specific module (e.g. gradoop-grala) which contains a DSL implementation that forwards calls to the internal API.