Added optional interpolation of LAI from monthly to daily.  This feature
is controlled by the new INTERP_LAI option in the global parameter file.
If INTERP_LAI is TRUE, VIC will assign the monthly LAI values supplied in
either the veg parameter file (if VEGPARAM_LAI is TRUE) or the veg library
file to the middle of each month and estimate daily LAI values via linear
interpolation.

Along with this feature we have added a new output variable, OUT_LAI,
so that the user can easily check the LAI values present in a grid cell
on any given day.  Note that these reported values will be the area-weighted
average over all veg tiles within the cell, as with all other reported
quantities.