Fix integration pipeline

This MR fixes several issues with the integration pipeline.

1. !349 (closed).

2. Fixes an oversight introduced in !344 (merged). The issue was with expressions like:

   a: scalar float = CastFunc(b: vector float, float)

   The mentioned MR used get_type_of_expression to determine whether an expression will be vectorized. That works in most cases due to type collation but fails for CastFuncs, which are always vectorized. This MR replaces get_type_of_expression by a small helper function that checks whether an expression is entirely scalar or requires additional vector casts.

pystencils/gpu/indexing.py

@@ -398,7 +398,8 @@ def _loop_ctr_assignments(loop_counter_symbols, coordinates, iteration_space):
     loop_ctr_assignments = []
     for loop_counter, coordinate, iter_slice in zip(loop_counter_symbols, coordinates, iteration_space):
         if isinstance(iter_slice, slice) and iter_slice.step > 1:
-            loop_ctr_assignments.append(SympyAssignment(loop_counter, coordinate * iter_slice.step))
+            offset = (iter_slice.step * iter_slice.start) - iter_slice.start
+            loop_ctr_assignments.append(SympyAssignment(loop_counter, coordinate * iter_slice.step - offset))
         elif iter_slice.start == iter_slice.stop:
             loop_ctr_assignments.append(SympyAssignment(loop_counter, 0))
         else: