Added per-type vertical height normalization

Added a namelist variable (special_vert_norm_height in &location_nml)
that allows the user to define different vertical normalization values
for each of the special localization types defined in
special_localization_obs_types in &assim_tools_nml. This is only implemented
for the case where the vertical localization coordinate is height
(when vert_localization_coord=3 in &model_nml).

If no obs types are defined in special_localization_obs_types, or if there
is not a corresponding value in special_vert_norm_height for each obs type
in special_localization_obs_types, then the obs type uses vert_normalization_height
in &location_nml.

If maintain_original_vert = .true. in &location_nml, then
special_vert_norm_height is *disregarded* and the default vertical localization
length (defined with vert_normalization_height and cutoff) is kept constant
across all obs types in special_localization_obs_types.