[Bug] Incorrect root fraction calculation

[tbohn]

• VIC version: 5.1.0.rc1
• Problem Description:
  -- In some cases, calc_root_fraction() computes substantially different root fractions from those of prior releases. This leads to substantially different rates of transpiration.
  -- This change arose in PR improve floating point comparisons in vic_init.c #701, which included some general cleanup fixes, including a fix to calc_root_fraction.c that was intended to fix problems with occasional cases of infinite loops in the algorithm. The fix did eliminate infinite loops, but changed the apportioning of root fractions to soil layers. One line in particular seems to have caused the change in apportioning: zone ++; was inserted on line 137.
• Proposed Solution:
  -- Fix the algorithm to apportion correctly and also end the loop correctly.

[dgergel]

Closed via #794