Only generate points for atoms of interest in volume calculations

[tooooby]

For %Vbur calculations, memory use and speed could be improved by only generating points for atoms which intersect with the probe sphere. This should be determinable by a simple calculation: if the distance of their center points is less than the sum of their radii.

This should be determined before fitting the grid, and should generally allow for much smaller grids to be required (depending on the radii you are probing).