add return type to DataReader to avoid type mismatch

Resolves type mismatch.
With moliere 0.1.3-SNAPSHOT and opensplice-mobile 1.1.1:
[error] /app/dscript/controllers/EntityManagerActor.scala:97: type mismatch;
[error]  found   : Unit
[error]  required: org.omg.dds.sub.DataReader[?]
[error]           val wdr = new WebReader(dr)