Add Dir Both where we want an arrow tail + Remove NoArrow tails

Apparently it's a graphviz 'feature' to not draw arrow tails unless 'Dir Both' is specified.
Compute aggregations
Use aggChild/aggParent instead of aggSrc/aggTgt
Disable aggregates

We first need to establish the correct conditions that determine whether a relation is an aggregate. With the currently implemented conditions, some relations that are clearly not aggregates will nevertheless be considered as such.
Merge pull request #15 from AmpersandTarski/show-aggregations-in-logical-data-model

Show aggregations in logical data model

Code is currently disabled.