Support Reductions

pystencils supports sympy Sum like this:

sum = sp.Sum(sp.abc.k, (sp.abc.k, 1, 100)) will be printed as:

{
   for (int64_t ctr_0 = 0; ctr_0 < _size_x_0; ctr_0 += 1)
   {
      _data_x[_stride_x_0*ctr_0] = [&]() {
         double sum = (double) 0;
         for ( int k = 1.0; k <= 100.0; k += 1 ) {
            sum += k;
         }
         return sum;
      }();
   }
}

This is C++ code and additionally, at the moment, it bypasses the type system a bit. Thus this should be reimplemented.