PK_ERROR_code_t     PK_SURF_eval_curvature
(
--- received arguments ---
PK_SURF_t           surf,                  --- surface
PK_UV_t             uv,                    --- parametric position

--- returned arguments ---
PK_VECTOR1_t *const normal,                --- surface normal
PK_VECTOR1_t *const principal_direction_1, --- principal direction 1
PK_VECTOR1_t *const principal_direction_2, --- principal direction 2
double       *const principal_curvature_1, --- principal curvature 1
double       *const principal_curvature_2  --- principal curvature 2
)


This function calculates the normal, principal directions and principal
curvatures of a surface at a given parametric position.


Specific Errors:
    PK_ERROR_eval_failure          failed to evaluate posn. or normal at uv
    PK_ERROR_bad_parameter         uv not suitable for surf
    PK_ERROR_at_singularity        uv is at a singularity of surf


The normal and principal directions are returned as unit vectors.

If the principal curvatures are equal then the principal directions are not
significant.

The surface must be capable of passing the checks imposed by PK_GEOM_check.