SWIG: introduce a typemap to use Python sequences instead of R3

Wrap/Swig/r3_typemap.i

+// typemap R3 to accept a Python R3 instance or a Python sequence
+
+%ignore BA_SWIG_getArg_R3;
+
+%inline
+%{
+
+// convert a given Python R3 instance or a Python sequence to a C++ R3 instance
+int BA_SWIG_getArg_R3(PyObject* input, swig_type_info* pTypeInfo,
+                      void*& r3_ptr, R3& r3_tmp)
+{
+    // if the input is already an R3 instance, merely extract the pointer
+    int res = SWIG_ConvertPtr(input, &r3_ptr, pTypeInfo, 0);
+    if (SWIG_IsOK(res)) {
+        return 1;
+    } else if (PySequence_Check(input) && PySequence_Size(input) == 3) {
+        // check if the input is a Python sequence of length 3
+
+        // extract the elements
+        PyObject *x = PySequence_GetItem(input, 0);
+        PyObject *y = PySequence_GetItem(input, 1);
+        PyObject *z = PySequence_GetItem(input, 2);
+
+        // check that all elements of the Python sequence are numbers
+        if (!(PyNumber_Check(x) && PyNumber_Check(y) && PyNumber_Check(z))) {
+            Py_XDECREF(x);
+            Py_XDECREF(y);
+            Py_XDECREF(z);
+            return -1;
+        }
+
+        r3_tmp = R3(PyFloat_AsDouble(x), PyFloat_AsDouble(y), PyFloat_AsDouble(z));
+        Py_XDECREF(x);
+        Py_XDECREF(y);
+        Py_XDECREF(z);
+
+        return 2;
+    } else {
+        return -2;
+    }
+}
+
+%}
+
+// SWIG typemap for input: Python iterable/R3 instance to R3&
+
+// NOTE: Mapping a non-const R3 argument should be avoided, since in that case,
+// the caller might expect that the input argument be modified,
+// and with Python sequences (used in place of R3), this modification is not performed.
+
+// NOTE: This method works only for functions or methods which are not overloaded.
+// Overloaded functions or methods must be explicitly 'extended' to accept a
+// Python sequence instead of an R3 instance.
+
+%typemap(in) const R3& (R3 tmpR3) {
+    // if the input is already an R3 instance, merely extract the pointer
+    void* argp = 0;
+    swig_type_info* pTypeInfo_R3_ref = SWIG_TypeQuery("R3&");
+    const int res = BA_SWIG_getArg_R3($input, pTypeInfo_R3_ref, argp, tmpR3);
+
+    switch(res) {
+    case 1:
+        $1 = reinterpret_cast<R3*>(argp);
+        break;
+    case 2:
+        $1 = &tmpR3;
+        break;
+    case -1:
+        PyErr_SetString(PyExc_TypeError, "R3: Python sequence elements must be numbers");
+        SWIG_fail;
+        break;
+    default:
+        PyErr_SetString(PyExc_TypeError, "R3: expected an R3 instance or a Python sequence of 3 numbers");
+        SWIG_fail;
+    }
+}