VRPN fixes (runtime, compilation, and external linkage)

Alexis Giraultrequested to merge
alexis-girault/iMSTK:VRPN-fixes into master
  1. VRPN would not be found in FindVRPN.cmake on Linux if iMSTK_USE_OMNI was set to ON.
  2. A name issue in iMSTKConfig.cmake.in would prevent external projects to link against iMSTK
  3. An offset was applied on VRPN devices on the client side while there already was a way to set up offsets in the controller interface

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