gridloop1 in C; safety checks

  if (a->nd != 2 || a->descr->type_num != PyArray_DOUBLE) {
    PyErr_Format(PyExc_ValueError,
    "a array is %d-dimensional or not of type float", a->nd);
    return NULL;
  }
  nx = a->dimensions[0];  ny = a->dimensions[1];
  if (xcoor->nd != 1 || xcoor->descr->type_num != PyArray_DOUBLE ||
      xcoor->dimensions[0] != nx) {
    PyErr_Format(PyExc_ValueError,
    "xcoor array has wrong dimension (%d), type or length (%d)",
		 xcoor->nd,xcoor->dimensions[0]);
    return NULL;
  }
  if (ycoor->nd != 1 || ycoor->descr->type_num != PyArray_DOUBLE ||
      ycoor->dimensions[0] != ny) {
    PyErr_Format(PyExc_ValueError,
    "ycoor array has wrong dimension (%d), type or length (%d)",
		 ycoor->nd,ycoor->dimensions[0]);
    return NULL;
  }
  if (!PyCallable_Check(func1)) {
    PyErr_Format(PyExc_TypeError,
    "func1 is not a callable function");
    return NULL;
  }