[IMP-dev] Grid class

Here is a proposed grid class (I haven't even tried to compile it). It can map between real space and voxels.

The differences from the grid used in EMBed are: - The voxel index is actually a triple if ints. This makes computing a point in a voxel fast without the large tables used in EMBed. I think this is the right choice speed wise, but we can always test (and it is trivial to change). - the names are changed to be more consistent with IMP - the voxel data is a template argument - it is copyable - currently there is no resize function and I don't really know what would be meant by it and the obvious things involve lots of moving of data around.