Improve duplicate handling in sterics.occupied

[tooooby]

Currently, we convert the list of indices to a set and then back to a list to remove duplicates. This does not scale very well compared to a simple alternative: make a bit mask (each bit marking whether a point is occupied or not) and, as you are scanning out the sphere's, set the needed indices to true. This means there will be no duplicates as each value is restricted to true or false. The overall grid can then be indexed using this mask to make the occupied grid.