Redesign auto-generated ALARA geometry information to create zones with appropriate intervals

[gonuke]

Currently, each interval (entry in a volume input block) is assigned to a distinct zone (entry in a mat_loading input block). In this new workflow, three new features are needed in scripts/r2s/io/write_alara_geom.py:

• generate a "pseudo-" interval for each cell fraction of each mesh voxel in the volumes block
• assign each pseudo-interval to a zone that represents the MCNP cell in the volumes block
• generate only 1 zone per MCNP cell in the mat_loading block

[erelson]

For clarification purposes for this issue and #63.

Note that the ordering of volumes and fluxes must match.