Many basic 3D functions have finally been implemented.

These functions now have a proper implementation and test cases:

  - hpix_angles_to_3dvec
  - hpix_3dvec_to_angles
  - hpix_3dvec_to_ring_pixel
  - hpix_3dvec_to_nest_pixel
  - hpix_ring_pixel_to_3dvec
  - hpix_nest_pixel_to_3dvec

The new type 'hpix_3d_vector_t' has been introduced, together with
a few utility functions (hpix_vector_length and hpix_dot_product).