Improve ontology

[retog]

The ontology (https://github.com/fusepool/datalifecycle/blob/master/dlc-ontologies/src/main/resources/eu/fusepool/datalifecycle/ontologies/dlc.ttl) introduced in #49 is rather unclear. The basic concepts (taks, datasets, pipes (obsolete I think) are used in the description but never properly introduces. It helps when properties have a specified domain and range and the description clearly states how the object relates to the subject. @luigiselmi