Added the ability to account for sub-grid heterogeneity in water table
depth.  This is primarily applicable to boreal wetlands; the water
table depth distribution is hard-coded to represent typical boreal
boreal wetland microtopography.

The water table depth distribution is controlled by the parameter
RIDGE_FRACT (defined in vicNl_def.h), which is the area fraction of
the wetland covered by ridges or hummocks, and is set to 0.5.
Ridges and hummocks are assumed to be 50cm tall, and hollows and pools
are assumed to be 20cm deep, for a total range of 70cm in surface
elevation.  This is based on field observations in Eppinga et al
(2008).  The water table depth distribution is sampled at 5 locations:
1. saturated zone, 2. midpoint of exposed hollow, 3. lowest 1/3 of
ridge, 4. middle 1/3 of ridge, 5. highest 1/3 of ridge.

This feature is optional, controlled by the value of DIST_ZWT in the
global parameter file.  By default, DIST_ZWT is FALSE.

There are several new output variables associated with this feature:
  OUT_DISTZWT_ASAT:     Array of saturated fractional areas, one per
                        point on the water table distribution
  OUT_DISTZWT_SMOIST:   2-d array of soil moistures (water table
                        distribution node and soil layer)
  OUT_DISTZWT_ZWT:      Array of water table depths (assigning water
                        table to lowest unsaturated layer)
  OUT_DISTZWT_ZWT_LUMP: Array of water table depths (lumping all
                        layers' water into one column)
  OUT_DISTZWT_BFLOW:    Array of baseflows
  OUT_DISTZWT_RUNOFF:   Array of runoffs

When DIST_ZWT is TRUE, the non-distributed versions of these variables
(OUT_ASAT, etc) are now computed as the area-weighted averages of the
values in the OUT_DISTZWT* arrays.