#10 Improvements in relation modeling

[sporgenza]

Relation modeling is a bit cumbersome at the moment if only simple relations are needed (i.e. with no additional attributes), as it quickly gets confusing with large models at the moment.
In cases that just require simple relations it would be more convenient to have an extra connection type that could be used to define those simple relations simply exhibiting a direction and a name. A very useful plus would be a bidirectional relation with optionally a name for the start- and the endpoint.
A quick and dirty way to achieve this (with the current architecture) may be an intermediate step that derives one or two relation entities and corresponding connections (with generated names) from this new connection type during the model generation.