change min parts floating voxels logic

[mattwla]

I altered my local copy so I can specify a minimum amount of new worlds needed to go through with a spawn floating voxel world operation. If less than that amount is found, nothing happens. (see "minworlds" parameter in diff)

as a side note, you can also see some changes I made to hide the transition between the old voxel world having its floating voxels edited away, and a new voxel world spawning representing those falling voxels.

voxelphysics-diff.txt