Detector2D now parameterized through width and center in phi,alpha (#854)

Some code duplication in the GUI (Axis -> Span property and form), to be revised later.

Resolves #854 (closed).

CHANGELOG

@@ -4,8 +4,9 @@ BornAgain-22.0, in preparation
     * LambdaScan for time-of-flight offspec
     * Datafield::flat flattens out axis of size 1
    > API:
-    * Detector2D replaces RectangularDetector and SphericalDetector. Nominal axes are equidistant,
-      but scattering is computed for the actual phi, alpha of each pixel.
+    * Detector2D replaces RectangularDetector and SphericalDetector. Parameterized through
+      span and position of center. Nominal axes are equidistant, but scattering is computed
+      for the actual phi, alpha of each pixel.
     * Offspec analyzer is set for detector, not scan(#651)
     * FitObjective::addSimulationAndData has been replaced by function addFitPair,
       which takes experimental data in form of a Datafield object instead of a NumPy array