Extension to field read extraction

With add_subexpressions_for_field_reads it is possible to extract field reads from the kernel and put them in individual assignements. For mixed precision kernels, however, it is useful if the lhs of this new assignement is of given type. This isolates casts and prevents calculations in the data type of the stored values

pystencils_tests/test_simplifications.py

@@ -136,7 +136,7 @@ def test_add_subexpressions_for_field_reads():
 
     main = [Assignment(s[0, 0](0), 3 * v[0, 0](0)),
             Assignment(s[0, 0](1), 10 * v[0, 0](1))]
-    
+
     ac = AssignmentCollection(main, subexpressions)
     assert len(ac.subexpressions) == 0
     ac2 = add_subexpressions_for_field_reads(ac)