MCNP: track() is called too often when using a weight window mesh (performance)

[gonuke]

Describe the Bug

When using a weight window mesh, native MCNP does not call track if the event in hstory is a mean-free-path weight window mesh event. It just decrements the distance to the surface and continues on.

In DAGMC, we do call track even if the event is a mean-free-path weight window mesh event, causing at least one extra expensive call to track, and possibly more.

We should revisit this logic to prevent excessive calls to track.