Draft: Apply detector resolution to offspec simulation (after !2896)

Only alpha resolution is accepted, because we integrate over phi anyway.

• zero resolution is allowed
• offspec instrument accepts resolution in GUI
• DetectorItem is a base for ScatteringDetectorItem and OffspecDetectorItem-

Resolves #1113

Base/Math/Functions.cpp

@@ -41,6 +41,9 @@ double Math::Gaussian(double x, double average, double std_dev)
 
 double Math::IntegratedGaussian(double x, double average, double std_dev)
 {
+    if (std_dev <= std::numeric_limits<double>::min())
+        return Math::Heaviside(x - average);
+
     double normalized_x = (x - average) / std_dev;
     static double root2 = std::sqrt(2.0);
     return (gsl_sf_erf(normalized_x / root2) + 1.0) / 2.0;