Delete asterisk in derivative methods of IdentityFunction

There has been an errant asterisk in the arguments of the (trivial) derivative method functions for IdentityFunction for an unknown amount of time. These methods did not work, and apparently were never tested.

HARK/interpolation.py

@@ -611,9 +611,9 @@ def derivative(self, *args):
         Returns the derivative of the function with respect to the first dimension.
         """
         if self.i_dim == 0:
-            return np.ones_like(*args[0])
+            return np.ones_like(args[0])
         else:
-            return np.zeros_like(*args[0])
+            return np.zeros_like(args[0])
 
     def derivativeX(self, *args):
         """
@@ -625,9 +625,9 @@ def derivativeX(self, *args):
         else:
             j = 0
         if self.i_dim == j:
-            return np.ones_like(*args[0])
+            return np.ones_like(args[0])
         else:
-            return np.zeros_like(*args[0])
+            return np.zeros_like(args[0])
 
     def derivativeY(self, *args):
         """
@@ -639,9 +639,9 @@ def derivativeY(self, *args):
         else:
             j = 1
         if self.i_dim == j:
-            return np.ones_like(*args[0])
+            return np.ones_like(args[0])
         else:
-            return np.zeros_like(*args[0])
+            return np.zeros_like(args[0])
 
     def derivativeZ(self, *args):
         """
@@ -653,9 +653,9 @@ def derivativeZ(self, *args):
         else:
             j = 2
         if self.i_dim == j:
-            return np.ones_like(*args[0])
+            return np.ones_like(args[0])
         else:
-            return np.zeros_like(*args[0])
+            return np.zeros_like(args[0])
 
     def derivativeW(self, *args):
         """
@@ -669,9 +669,9 @@ def derivativeW(self, *args):
                 False
             ), "Derivative with respect to W can't be called when n_dims < 4!"
         if self.i_dim == j:
-            return np.ones_like(*args[0])
+            return np.ones_like(args[0])
         else:
-            return np.zeros_like(*args[0])
+            return np.zeros_like(args[0])
 
 
 class ConstantFunction(MetricObject):