Unit test for vpgl_poly_radial_distortion constructor.

Currently segfaults if pointer is NULL.
set_coefficients tests for NULL pointer before dereferencing.

Associated unit test now passes.
Set has_derivative flag to true in vpgl_poly_radial_distortion constructor

Constructors which receive coefficients in vcl_vectors should also set the
parent's has_derivative flag to true.
Adding read-only and read-write coefficient accessor methods
explicitly instantiating lens models with float