Fix step depth in grahamGetCHSteps

If we have 3 points and pass 1 to 'g' then 2 steps will be computed.
That means we have to substract 2 from the total length in the call
to 'g'.

Algorithms/ConvexHull.hs

@@ -69,7 +69,7 @@ grahamGetCH vs = f . grahamSort $ vs
 -- |Compute all steps of the graham scan algorithm to allow
 -- visualizing it.
 grahamGetCHSteps :: [PT] -> [[PT]]
-grahamGetCHSteps vs = reverse . g $ (length . grahamGetCH $ vs)
+grahamGetCHSteps vs = reverse . g $ (length vs - 2)
   where
     vs' = grahamSort vs
     g c