Added subclasses of spot location as it showed during review with coord IS that the possibility to define an intrinsic coordinate spotlocation was lost with 1.5

#24: changed base class of element part collection and added diagram to show inheritors of baseobject

#17: moved association of spotlocation from positioning net element to net element. That way also net connectors can be referenced by a spot location. also introduced new diagrams for better unterstanding and fixed some smaller issues that became visibile with the new focussed views

#23: removed LinearNetElement and NonLinearNetElement from the model as agreed with coord IS

#11: clean up. removed associatedNetElement subclass in order to clean up the way coordinates for linear locations are specified

#18: renamed measureToNext to distanceToNext in order to improve understandability

#20: Removed association between Network and NetworkResource. This was done to eliminate the possibility to have NetworkResources that do not belong to a LevelNetwork. If something belongs to a network is needs to be organized in a LevelNetwork.

#21: inverted the direction of the association between LinearPositioningSystem and LinearAnchorPoint and indicated that the collection needs to be odered. This was also the reason why the relationship needed to be inverted.

Resolve "Documentation of keepsOrientation"

Closes #3

See merge request !7

Resolve "Redundant references in railML 3.x topology"

Closes #10

See merge request !6

removed class relation and replaced it with class netrelation, which formerly was a subclass of relation.