Removal of length attribute from netElement

The length attribute introduced for netElements in 1.4 (#5 (closed)) shall be removed again. Reason is that the actual dimension is not clear and should be kept out of the RTM. If an implementer needs to specify a length, this can be done of course but it would be outside the RTM. Example for this would be railML where the length attribute of the netElement is specified on the level of the railML netElement sub class. Another point that lead to the removal requirement was that with the introduction of the netConnector a netElement subclass was introduced that always is point shaped and thus cannot have a length.