Create RelationDef subtypes for Relation implementations (e.g. SingleRowRelationDef)

[srfc]

Issue

• Make RelationDef abstract and use subtypes to define specific Relation implementations

Problem Description

• Currently a RelationDef can be used to define an arbitrary Relation implementation (SingleRowRelation, RowRelation, ColumnRelation, etc.)
• The semantic of a RelationDef therefore is merely a schema definition
• This leads to a mix of concerns when defining Dactors because only there it is defined what kind of Relation is actually instantiated from the RelationDef
• Proposed interface allows for clear definition syntax even when using various Relation types in a Dactor:

// Companion object
object Info extends SingleRowRelationDef("info") {
  // [...]
}
object Cast extends RowRelationDef("cast") { 
  // [...]
}

// Dactor
class Film(id: Int) extends Dactor(id) {
  override protected val relations: Map[RelationDef, MutableRelation] =
    createRelations(Set(Film.Info, Film.Cast)) // a single relation instantiation function!
  override def receive: Receive = Actor.emptyBehavior
}

• This will also make it easier to develop a DSL for data modelling