Fix to division by 0 in runoff.c, plus minor change to evap logic for in...

...ternal consistency.  This not only addresses issue #138, but also addresses issue #145 (nans on develop branch), since fixing #138 also removed the nans.

Note: I also noticed small water balance errors.  These were present before the fix and are still present, i.e. the fix did not cause them.  It is possible that these errors are related to the same overall code change (i.e., errors associated with available moisture and the movement of SPATIAL_FROST from user_def.h option to run-time option).  Moving SPATIAL_FROST to run-time required making layer moisture, temperature, evap, etc. into arrays over the frost_subareas dimension and summing over this dimension for all water balance operations, even when SPATIAL_FROST is FALSE.
Ted - earlier we had agreed to no longer track these changes in the comment header of each function, but instead use a sensible commit message (see issue #105, which has been closed). Could you remove these comment changes and re-issue the pull request? I'll close this one unmerged for now.