Clarify semantics of fancy integer division functions.

Closes #88 (closed).

The goal of these functions is to provide fast, fp-free functions for semi-complicated semantics on positive integers (The old implementation did not behave sensibly for negative input). This MR clarifies the names and docstrings of the fancy integer divisions to make it clear that negative input is not always expected. Furthermore, the implementation is changed such that no remainders or conditionals must be evaluated.

The old names are still available but issue deprecation warnings.

src/pystencils/sympyextensions/integer_functions.py

 import sympy as sp
 import warnings
+from pystencils.sympyextensions import is_integer_sequence
 
 
 class IntegerFunctionTwoArgsMixIn(sp.Function):
@@ -87,17 +88,22 @@ class round_to_multiple_towards_zero(IntegerFunctionTwoArgsMixIn):
         -8
     """
 
-    def _eval_op(self, arg1, arg2):
+    @classmethod
+    def eval(cls, arg1, arg2):
         from ..utils import c_intdiv
 
-        return c_intdiv(arg1, arg2) * arg2
+        if is_integer_sequence((arg1, arg2)):
+            return c_intdiv(arg1, arg2) * arg2
+
+    def _eval_op(self, arg1, arg2):
+        return self.eval(arg1, arg2)
 
 
 # noinspection PyPep8Naming
 class ceil_to_multiple(IntegerFunctionTwoArgsMixIn):
-    """For positive input, returns the next greater/equal integer divisible by
-    given divisor. The return value is implementation defined if either argument
-    is negative.
+    """For positive input, returns the next greater/equal integer divisible
+    by given divisor. The return value is unspecified if either argument is
+    negative.
 
     Examples:
         >>> ceil_to_multiple(9, 4)
@@ -108,17 +114,22 @@ class ceil_to_multiple(IntegerFunctionTwoArgsMixIn):
         12
     """
 
-    def _eval_op(self, arg1, arg2):
+    @classmethod
+    def eval(cls, arg1, arg2):
         from ..utils import c_intdiv
 
-        return c_intdiv(arg1 + arg2 - 1, arg2) * arg2
+        if is_integer_sequence((arg1, arg2)):
+            return c_intdiv(arg1 + arg2 - 1, arg2) * arg2
+
+    def _eval_op(self, arg1, arg2):
+        return self.eval(arg1, arg2)
 
 
 # noinspection PyPep8Naming
 class div_ceil(IntegerFunctionTwoArgsMixIn):
     """For positive input, integer division that is always rounded up, i.e.
-    `div_ceil(a, b) = ceil(div(a, b))`. The return value is implementation
-    defined if either argument is negative.
+    `div_ceil(a, b) = ceil(div(a, b))`. The return value is unspecified if
+    either argument is negative.
 
     Examples:
         >>> div_ceil(9, 4)
@@ -127,10 +138,15 @@ class div_ceil(IntegerFunctionTwoArgsMixIn):
         2
     """
 
-    def _eval_op(self, arg1, arg2):
+    @classmethod
+    def eval(cls, arg1, arg2):
         from ..utils import c_intdiv
 
-        return sp.sympify(c_intdiv(arg1 + arg2 - 1, arg2))
+        if is_integer_sequence((arg1, arg2)):
+            return c_intdiv(arg1 + arg2 - 1, arg2)
+
+    def _eval_op(self, arg1, arg2):
+        return self.eval(arg1, arg2)
 
 
 # Deprecated functions.