* Changes to DerivedMesh interface: DMGridData has been removed,
getGridData() now returns an array of CCGElem pointers. Also added
getGridKey() to initialize a CCGKey (implemented only by
CCGDerivedMesh.)
* PBVH: added BLI_pbvh_get_grid_key().
* A lot of code is affected, but mainly is just replacing
DMGridData.co, DMGridData.no, and sizeof(DMGridData) with the
CCG_*_elem functions, removing the reliance on grid elements of
exactly six floats.