Create a marching cubes example which corresponds to the documentation stub.

I do hope this is a desired and useful addition to the project which may mitigate the inevitable work involved in getting me up to speed on this codebase.

Note that there is a problem of currently unknown impact:

➜  build_vtkm ./examples/marching_cubes/MarchingCubes
vtk-m/vtkm/exec/serial/internal/TaskTiling.h:291:5: runtime error: call to function void vtkm::exec::serial::internal::TaskTilingSetErrorBuffer<vtkm::worklet::marchingcubes::ClassifyCell<float> const>(void*, vtkm::exec::internal::ErrorMessageBuffer const&) through pointer to incorrect function type 'void (*)(void *, const vtkm::exec::internal::ErrorMessageBuffer &)'
TaskTiling.h:31: note: void vtkm::exec::serial::internal::TaskTilingSetErrorBuffer<vtkm::worklet::marchingcubes::ClassifyCell<float> const>(void*, vtkm::exec::internal::ErrorMessageBuffer const&) defined here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior vtk-m/vtkm/exec/serial/internal/TaskTiling.h:291:5 in
vtk-m/vtkm/exec/serial/internal/TaskTiling.h:201:5: runtime error: call to function void vtkm::exec::serial::internal::TaskTilingSetErrorBuffer<(anonymous namespace)::ReverseInputToOutputMapWorklet const>(void*, vtkm::exec::internal::ErrorMessageBuffer const&) through pointer to incorrect function type 'void (*)(void *, const vtkm::exec::internal::ErrorMessageBuffer &)'
TaskTiling.h:31: note: void vtkm::exec::serial::internal::TaskTilingSetErrorBuffer<(anonymous namespace)::ReverseInputToOutputMapWorklet const>(void*, vtkm::exec::internal::ErrorMessageBuffer const&) defined here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior vtk-m/vtkm/exec/serial/internal/TaskTiling.h:201:5 in

I have seen this error in every example I've run, so it is not unique to this PR. But this does not mitigate it either.