FIX: Redefine slope of RampWaveform (#644)

* Redefine slope

* Modify docstring

* Fix docstring

pulser-core/pulser/waveforms.py

@@ -532,8 +532,8 @@ class RampWaveform(Waveform):
 
     Args:
         duration: The waveform duration (in ns).
-        start: The initial value (in rad/µs).
-        stop: The final value (in rad/µs).
+        start: The value (in rad/µs) at the initial sample.
+        stop: The value (in rad/µs) at the final sample.
     """
 
     def __init__(
@@ -566,7 +566,7 @@ def _samples(self) -> np.ndarray:
     @property
     def slope(self) -> float:
         r"""Slope of the ramp, in :math:`s^{-15}`."""
-        return (self._stop - self._start) / self._duration
+        return (self._stop - self._start) / (self._duration - 1)
 
     def change_duration(self, new_duration: int) -> RampWaveform:
         """Returns a new waveform with modified duration.

tests/test_waveforms.py

@@ -162,7 +162,7 @@ def test_custom():
 
 
 def test_ramp():
-    assert ramp.slope == 7e-3
+    assert np.isclose(ramp.slope, 7e-3, atol=1e-5)
 
 
 def test_blackman():