Adaptive multimodal Van Genuchten functions in hydrol.
This replaces the standard Mualem - Van Genuchten model with one
explicitly representing five pore size classes: inaccessible, bacterial,
micro, meso, and macro. These classes and the threshold pore radii come
from the KEYLINK model. For now, the pore size distribution (PSD) is
fixed and used once during the initialization of hydrol. However, most
preparations are already in place for having the PSD change in time. In
particular, hydrol_pixel_update duplicates some code from
hydrol_var_init. The duplicated bits are precisely the ones which will
need to be rerun when the PSD changes (hence 'update' in the name of the
routine). This is also why ks, mcs, and friends are now inout in
hydrol.
The modification is controlled by five parameters. OK_MULTIMOD_HYDROL (n
by default) turns the scheme on. Then, EXTRA_MACRO_PORES,
EXTRA_MESO_PORES, EXTRA_MICRO_PORES, and EXTRA_BACT_PORES (pore volumes
per soil volume, hence dimensionless) allow one to increase the porosity
in a chosen size class, for all pixels uniformly. Negative values are in
principle possible, but one must take serious case to avoid negative
pore volumes.
