WIP: Particle advection performance fixes
This branch has code to introduce the fast paths for structured grids again. The majority of the changes are
- Returning logical cells from the cell locators which use structured grids
- Not redundantly calculating parametric coordinates in case of uniform grid cell locator.
- Using a special interpolation path for structured grids which does not use
- In some places, parameters were being passed as values instead of references.
The changes are not just particle advection specific, but touch the interfaces of the
vktm::exec::CellLocators as well.
The slowdown compared to the previous versions now is on the order of 1.12x which we feel with the new code base is acceptable.