Apply suggestions from code review

Comments and a minor refactor for consistency.

orchard_group_hash.py

@@ -99,10 +99,7 @@ def map_to_curve_simple_swu(u):
     e1 = x1 == Fp(0)
     x1 = x1 + Fp(1)
 
-    if e1:
-        x1 = c2
-    else:
-        x1 = x1
+    x1 = c2 if e1 else x1    # If (tv1 + tv2) == 0, set x1 = -1 / Z
 
     x1 = x1 * c1      # x1 = (-B / A) * (1 + (1 / (Z^2 * u^4 + Z * u^2)))
     gx1 = x1.exp(2)

orchard_iso_pallas.py

@@ -97,6 +97,7 @@ class Point(object):
         elif a.is_identity:
             return self
         else:
+            # Hüseyin Hışıl. “Elliptic Curves, Group Law, and Efficient Computation”. PhD thesis.
             # <https://core.ac.uk/download/pdf/10898289.pdf> section 4.1
             (x1, y1) = (self.x, self.y)
             (x2, y2) = (a.x, a.y)
@@ -119,6 +120,7 @@ class Point(object):
         if self.is_identity:
             return self
 
+        # Hüseyin Hışıl. “Elliptic Curves, Group Law, and Efficient Computation”. PhD thesis.
         # <https://core.ac.uk/download/pdf/10898289.pdf> section 4.1
         λ = (Fp(3) * self.x * self.x + PALLAS_ISO_A) / (self.y + self.y)
         x3 = λ*λ - self.x - self.x