use "approximate" formula in getRayCoord for small angles, this is numerically more accurate than the mathematically exact expression. Set h1max to 1e10, but it could be even bigger.
Make Gyoto work for large distances:
 * use mathematical approximation for small angles in Screen::getRayCoord
   which numerically more accurate than the exact trigonometry, this makes
   Gyoto work for the Galactic Center case without using the dmax rescaling
   trick;
 * make dmax and h1max default to DBL_MAX;
 * in KerrBL::myrk4_adaptive(), set difftol to 1e-6 (this makes Gyoto more
   reliable for an object far behind the BH).