Changed the definition of TransformationSrv

A TransformationSrv provides an arbitrary transformation,
while PoseSrv is meant to represent the pose of a system or
subsystem (i.e. the transformation from the world
to a frame of reference). TransformationSrv cannot therefore
provide PoseSrv by default.
Merge pull request #3 from goldhoorn/transformer

TransformationSrv should not be a PoseSrv by default