Material method to remove impurities below a given tolerance from material definitions

Impurities oftentimes have a negligible effect on particle transport, so including them in transport definitions is wasteful from a memory perspective. This necessitates the ability to remove impurities (below a certain atom fraction or mass fraction) from a material before printing.

This can be achieved with a method that looks like:

pyne.Material.remove_impurities(threshold=10E-5, threshold_type='atom')