Implemented remesher: behavior at zero densities

Due to some implementation error, the remeshing algorithm fails to accept zero density regions as decently remeshed.
The offending line (in mesher.py::get_recursive_remesh line 779):
`if (maxval-minval)<threshold*(minval+maxval):` 
I would need to check whether just replacing the less than sign with less or equal does not break anything else.