CellSetExplicit::BuildConnectivity should not require specifying a device adapter

Currently the prototype for CellSetExplicit::BuildConnectivity is as follows.

  template<typename Device, typename FromTopology, typename ToTopology>
  VTKM_CONT
  void BuildConnectivity(Device, FromTopology, ToTopology) const

The problem is that you have to specify a particular device. Instead, it would be better if BuildConnectivity were defined as follows.

  template<typename FromTopology, typename ToTopology>
  VTKM_CONT
  void BuildConnectivity(
      FromTopology,
      ToTopology,
      vtkm::cont::RuntimeDeviceTracker tracker = vtkm::cont::GetGlobalRuntimeDeviceTracker()) const

Ideally, this version of BuildConnectivity would be built as part of the vtkm_cont library, at least for common instantiations of CellSetExplicit.