Change CellLocatorTwoLevelUniformGrid to use the new CellLocator API.

We had agreed to add a standard API for all CellLocators and PointLocators, which required all the cell locators to extend from the interface vtkm::cont::CellLocator, and all point locators to extend from the interface vtkm::cont::PointLocators suggested in the discussion #224

This is an issue to change the existing cell locator CellLocatorTwoLevelUniformGrid to use the new interface.