Beam: some setters rm from Py

Device/Beam/Beam.h

@@ -41,10 +41,6 @@ public:
     static Beam horizontalBeam();
 
     //... Setters:
-    void setIntensity(double intensity);
-    void setWavelength(double wavelength);
-    void setInclination(double alpha);
-    void setAzimuthalAngle(double value);
     void setFootprintFactor(const IFootprintFactor& shape_factor);
     void setPolarization(R3 polarization);
 
@@ -56,6 +52,12 @@ public:
         return "Beam";
     }
 
+    //... Setters:
+    void setIntensity(double intensity);
+    void setWavelength(double wavelength);
+    void setInclination(double alpha);
+    void setAzimuthalAngle(double value);
+
     std::vector<const INode*> nodeChildren() const override;
 
     //... Getters

hugo/content/py/instr/beam/_index.md

@@ -5,4 +5,18 @@ weight = 20
 
 ## Beam model
 
+The incident beam must be specified for scattering simulations with 2D detector
+(SAS and GISAS).
+For other simulations, the beam is set internally according to the
+given [scan](../scan) model.
+
+To create a beam, use
+```python
+ba.InBeam(intensity, wavelength, alpha, phi=0)
+```
+For the wavelenth, use unit multiplier `*nm` or `*angstrom`.
+For the glancing angle $\alpha\_\text{i}$, use unit multiplier `*deg`.
+The azimuthal angle $\varphi\_\text{i}$ can be omitted,
+as in most applications the default value 0 is pertinent.
+
 {{% children  %}}