template types should not start with underscore and a uppercase character

The c++ standard states:

all identifiers that contain a double underscore __ in any position and each identifier that begins with an underscore followed by an uppercase letter is always reserved and all identifiers that begin with an underscore are reserved for use as names in the global namespace.

We should either use _<lowercase> or better not use _ as the starting character.

Currently the following classes violate this:

/home/robert/Work/vtkm/src/vtkm/cont/ArrayHandleGroupVec.h:
   33  {
   34  
   35: template <typename _SourcePortalType, vtkm::IdComponent _NUM_COMPONENTS>
   36  class VTKM_ALWAYS_EXPORT ArrayPortalGroupVec
   37  {

/home/robert/Work/vtkm/src/vtkm/cont/internal/ArrayPortalFromIterators.h:
  213  /// the portal wrapped in an \c IteratorFromArrayPortal.
  214  ///
  215: template <typename _IteratorType>
  216  class ArrayPortalToIterators<vtkm::cont::internal::ArrayPortalFromIterators<_IteratorType>>
  217  {

/home/robert/Work/vtkm/src/vtkm/internal/Invocation.h:
   34  /// worklet. \c Invocation is a class that manages all these types.
   35  ///
   36: template <typename _ParameterInterface,
   37            typename _ControlInterface,
   38            typename _ExecutionInterface,

/home/robert/Work/vtkm/src/vtkm/worklet/Keys.h:
   75  /// creating a different \c Keys structure for each \c Invoke.
   76  ///
   77: template <typename _KeyType>
   78  class VTKM_ALWAYS_EXPORT Keys
   79  {
   ..
  271  };
  272  
  273: template <typename _KeyType>
  274  VTKM_CONT Keys::Keys() = default;
  275  }