ENH: add BandMathX expression in bonus

app/otbHarmonizer.cxx

@@ -404,6 +404,8 @@ private:
     FloatVectorImageType::PixelType gain, offset;
     gain.SetSize(refImage->GetNumberOfComponentsPerPixel());
     offset.SetSize(refImage->GetNumberOfComponentsPerPixel());
+    std::stringstream exp;
+    exp << "{";
     for (unsigned int band = 0 ; band < refImage->GetNumberOfComponentsPerPixel() ; band++)
     {
       float meanY = m_StatsFilter->GetMeans().at(band)[0][0];
@@ -417,9 +419,20 @@ private:
 
       otbAppLogINFO("Band " << band << " gain: " << b1 << " bias: " << b0);
 
+      if (band>0)
+        exp << ";";
+      exp << "im1b" << band << "*" << b1;
+      if (b0>0)
+        exp << "+" << b0;
+      else
+        exp << "" << b0;
+
       gain[band] = b1;
       offset[band] = b0;
     }
+    exp << "}";
+
+    otbAppLogINFO("BandMathX expression: " << exp.str());
 
     if (this->HasValue("out"))
     {