Simplify the grammar of the ROS model DSL

[ipa-nhg]

This PR simplifies the grammar to describe a ROS package and a node.

For example, the ros driver for the sick s300 scanner using the old version of the ros model looks:

PackageSet { package {
  CatkinPackage cob_sick_s300 { artifact {
    Artifact cob_sick_s300 { 
      node Node { name cob_sick_s300 
        publisher { 
          Publisher { name scan message "sensor_msgs.LaserScan" } , 
          Publisher { name diagnostics message "diagnostic_msgs.DiagnosticArray" 
      }}}}
}}}}

With the changes proposed here, the same node can be defined as:

PackageSet {
  CatkinPackage cob_sick_s300 {
    Artifact cob_sick_s300 {
      Node { name cob_sick_s300
        Publishers { 
          Publisher { name scan message "sensor_msgs.LaserScan" } , 
          Publisher { name diagnostics message "diagnostic_msgs.DiagnosticArray" 
    }}}}
  }
}

The proposed version of the de.fraunhofer.ipa.ros.xtext is backward compatible, i.e. support also models created following the old version of the grammar. The parts of the code to be deleted, in a future, to support only the new grammar are marked with the comment "to be deprecated"

[ipa-nhg]

Superseded by #125