BUG interpolate properly in offset_by (#75)

* BUG interpolate properly in offset_by

* Test offset by/to

attune/curve/_base.py

@@ -375,7 +375,7 @@ def offset_by(self, dependent, amount):
         """
         # offset
         self.dependents[dependent].positions += amount
-        self.dependents[dependent].interpolate()
+        self.interpolate()
 
     def offset_to(self, dependent, destination, setpoint, setpoint_units="same"):
         """Offset a dependent such that it evaluates to `destination` at `setpoint`.

tests/Curve/offset.py

@@ -0,0 +1,26 @@
+import attune
+import numpy as np
+
+
+def test_offset_by():
+    d0 = attune.Dependent(np.linspace(-5, 5, 20), "d1")
+    s0 = attune.Setpoints(np.linspace(1300, 1400, 20), "s", "wn")
+    c0 = attune.Curve(s0, [d0], "c1")
+    c1 = c0.copy()
+
+    c1.offset_by("d1", 1.0)
+
+    np.testing.assert_allclose(c1["d1"][:], c0["d1"][:] + 1.0)
+    np.testing.assert_allclose(c1.setpoints[:], c0.setpoints[:])
+
+
+def test_offset_to():
+    d0 = attune.Dependent(np.linspace(-5, 5, 20), "d1")
+    s0 = attune.Setpoints(np.linspace(1300, 1400, 20), "s", "wn")
+    c0 = attune.Curve(s0, [d0], "c1")
+    c1 = c0.copy()
+
+    c1.offset_to("d1", 1.0, 1300)
+
+    np.testing.assert_allclose(c1["d1"][:], np.linspace(1, 11, 20))
+    np.testing.assert_allclose(c1.setpoints[:], c0.setpoints[:])