Section Surface Generic data Stucture for laws, rho, etc.
You can define any type of law (not just constant, linear, and
cubic), by using the UF_MODL_parm_t structure.

For the UF_MODL_ask_section_surface function, if the section
surface queried uses a constant, linear, or cubic law, then those
types still return as constant, linear, or cubic. However, if you use
any other type of law, it is noted as "UF_MODL_secsrf_general".
The data for that law appears in the data_values.general_law field of the
UF_MODL_secsrf_param structure.