Removed extraneous whitespace from MCNP geom input generation function.

There are 3 sections of an MCNP input file that are each delimited by a blank new line. Previously, ```mesh_to_geom``` added a blank new line at the end of the material specifications. When creating a full input file, the rest of the data cards need to follow immediately after the material cards in this case, without a new line interruption. This PR removes this interruption.