Add sign function and replace cases where most appropriate

copysign is usually faster, but won't necessarily handle zero cases
the same as sign(). In most cases this is fine, but in others, in may
be preferable to use sign.

coloraide/algebra.py

@@ -71,6 +71,14 @@
 ################################
 # General math
 ################################
+def sign(x: float) -> float:
+    """Return the sign of a given value."""
+
+    if x and x == x:
+        return x / abs(x)
+    return x
+
+
 def order(x: float) -> int:
     """Get the order of magnitude of a number."""
 

coloraide/temperature/ohno_2013.py

@@ -224,7 +224,7 @@ def to_cct(
         x = (dp ** 2 - dn ** 2 + l ** 2) / (2 * l)
         t = tp + (tn - tp) * (x / l)
         vtx = vp + (vn - vp) * (x / l)
-        sign = math.copysign(1, v - vtx)
+        sign = alg.sign(v - vtx)
         duv = (dp ** 2 - x ** 2) ** (1 / 2) * sign
 
         # Parabolic solution