Detector2D::indexOfSpecular newly implemented (#820)

This resolves #820 (closed).

Device/Detector/Detector2D.cpp

@@ -62,10 +62,13 @@ Pixel* Detector2D::createPixel(size_t index) const
 
 size_t Detector2D::indexOfSpecular(const Beam& beam) const
 {
-    double alpha = beam.alpha_i();
-    double phi = beam.phi_i();
-    if (alpha + phi < 0)
-        return -1; // issue #820
-    // return getGlobalIndex(
+    const double alpha = beam.alpha_i();
+    const double phi = beam.phi_i();
+
+    const double u = tan(phi);
+    const double v = hypot(1., u * u) * tan(alpha);
+
+    if (axis(0).rangeComprises(u) && axis(1).rangeComprises(v))
+        return getGlobalIndex(axis(0).closestIndex(u), axis(1).closestIndex(v));
     return totalSize();
 }