Fix SO2/SE2.interp1(), which raised NameError for every input

The fix for #33 dropped the `start` local from interp1() but only replaced its
two uses in the N == 3 branch, so the SO(2)/SE(2) branch has referenced an
undefined name ever since:

    SE2(1, 2, 0.3).interp1(0.5)   # NameError: name 'start' is not defined

#33 did report it for both SE2 and SE3.  Pass None like the N == 3 branch does;
trinterp2() already treats a None start as the identity.  interp1() had no test
coverage for either dimension, hence the four years.

Author: binggao1230

spatialmath/baseposematrix.py

@@ -522,10 +522,10 @@ def interp1(self, s: float = None) -> Self:
             # SO(2) or SE(2)
             if len(s) > 1:
                 assert len(self) == 1, "if len(s) > 1, len(X) must == 1"
-                return self.__class__([smb.trinterp2(start, self.A, s=_s) for _s in s])
+                return self.__class__([smb.trinterp2(None, self.A, s=_s) for _s in s])
             else:
                 return self.__class__(
-                    [smb.trinterp2(start, x, s=s[0]) for x in self.data]
+                    [smb.trinterp2(None, x, s=s[0]) for x in self.data]
                 )
         elif self.N == 3:
             # SO(3) or SE(3)

tests/test_pose2d.py

@@ -466,6 +466,23 @@ def test_interp(self):
         array_compare(T1.interp(T2, s=0.5, shortest=False), SE2(0, 0, 0.05))
         array_compare(T1.interp(T2, s=0.5, shortest=True), SE2(0, 0, -math.pi + 0.05))
 
+    def test_interp1(self):
+        # interpolate from the identity pose
+        TT = SE2(2, -4, 0.6)
+        array_compare(TT.interp1(0), SE2())
+        array_compare(TT.interp1(1), TT)
+        array_compare(TT.interp1(0.5), SE2(1, -2, 0.3))
+
+        z = TT.interp1([0, 0.5, 1])
+        self.assertEqual(len(z), 3)
+        array_compare(z[2], TT)
+
+        R = SO2(0.6)
+        array_compare(R.interp1(0), SO2())
+        array_compare(R.interp1(1), R)
+        array_compare(R.interp1(0.5), SO2(0.3))
+        self.assertEqual(len(SE2([TT, TT]).interp1(0.5)), 2)
+
     def test_miscellany(self):
         TT = SE2(1, 2, 0.3)