fixed bug in refract_paraxial:
requested to merge 314-paraxial-surface-does-not-use-any-kind-of-isometry-before-letting-the-rays-refract into 310-nice-set-of-stretcher-examples
self.dir was divided bei sel.dir.z, which inverts the direction for the case of propaation into negative z. Changed to division by self.dir.z.abs()
Closes #314 (closed)