Gan_Vector *pvx;
pvx = gan_vec_alloc(3);
gan_vec_fill_va ( pvx, 3, 2.0, 3.0, 4.0 );
gan_vec3_from_vec_q ( pvx, &v3x );
or
v3x = gan_vec3_from_vec_s ( pvx );
fill v3x with the same values as the general vector.
Calling these functions with a general vector pvx not having
the same size as the fixed size vector is an error.
Error detection: The conversion routines return the pointer to the filled fixed size vector, or NULL on failure, invoking the Gandalf error handle in the latter case.