Fixes for buffers in loops with step size > 1

This MR introduces some additions and fixes for generating CPU loops with step sizes > 1:

• The CPU create_kernel function now exposes a flag to disable the double field write check
• Rewrote get_base_buffer_index to use pure integer arithmetic, and corrected the computation of the buffer base index to correctly incorporate loop step sizes. Added test case to check correctness.
• Added rudimentary evalf functionality to integer division sympy function int_div (its absence lead to an infinite recursion during code generation).
• Added correct printing of integer-typed expressions in CustomSympyPrinter._typed_number.

pystencils/backends/cbackend.py

@@ -529,6 +529,14 @@ class CustomSympyPrinter(CCodePrinter):
             return res + '.0f' if '.' not in res else res + 'f'
         elif dtype.numpy_dtype == np.float64:
             return res + '.0' if '.' not in res else res
+        elif dtype.is_int():
+            tokens = res.split('.')
+            if len(tokens) == 1: 
+                return res
+            elif int(tokens[1]) != 0:
+                raise ValueError(f"Cannot print non-integer number {res} as an integer.")
+            else:
+                return tokens[0]
         else:
             return res