Add contrib scripts.

In addition to creating a new directory for contributing scripts, a
first contribution is included.

`finish_dagmc_geom.bash` performs imprint/merge/graveyard on a
well-formed geometry and is described in the READEM.md file.
Move contrib scripts since this first one is probably applicable to all physics.
Merge pull request #119 from svalinn/add_first_contrib_scripts

Add first contrib scripts