Fix to division by 0 in runoff.c, plus minor change to evap logic for internal consistency.  This not only addresses issue #138, but also addresses issue #145 (nans on develop branch), since fixing #138 also removed the nans.
Fix for issues #138 and #145.

In runoff.c, added check to handle case when sum_liq = 0.  Removed print of warning when evap_sum != 0 (this in itself is not an error) and added update of layer[lindex].evap to equal the sum of evap[lindex][frost_area] over all frost_areas after adjustment for avail_liq to ensure consistency.
Merge pull request #148 from tbohn/develop

Fix for issues #138 and #145.