disallow inf lower bound, -inf upper bound

If the lower bound is `inf`, no integer number will satisfy the lower bound,
which implies the array must have size zero, in which case the bounds are
useless. This patch disallows `inf` lower bounds and `-inf` upper bounds.

nutils/evaluable.py

@@ -989,8 +989,8 @@ def _intbounds(self):
             return value, value
         else:
             lower, upper = self._intbounds_impl()
-            assert isinstance(lower, int) or lower == float('-inf') or lower == float('inf')
-            assert isinstance(upper, int) or upper == float('-inf') or upper == float('inf')
+            assert isinstance(lower, int) or lower == float('-inf')
+            assert isinstance(upper, int) or upper == float('inf')
             assert lower <= upper
             return lower, upper