SampleToPython: import bornagain at the beginning of the produced Python script

Sim/Export/SampleToPython.cpp

@@ -537,7 +537,9 @@ std::string SampleToPython::sampleCode(const MultiLayer& sample)
         objHandler.insertModel("crystal", x);
 
     // clang-format off
-    return "def get_sample():\n"
+    return "import bornagain as ba\n"
+        "from bornagain import deg, nm, R3\n\n"
+        "def get_sample():\n"
         + defineMaterials(matHandler)
         + defineFormFactors(objHandler)
         + defineParticles(objHandler, matHandler)