clip values returned by get_cos() to remmove rounding errors (#139)

Author: YAY-C

qstack/qml/slatm.py

@@ -136,7 +136,7 @@ def get_cos(a, b, c):
         """
         v1 = r[a] - r[b]
         v2 = r[c] - r[b]
-        return v1 @ v2 / (dist[a,b] * dist[b,c])
+        return np.clip(v1 @ v2 / (dist[a,b] * dist[b,c]), -1, 1)
 
     theta1 = np.pi + theta0
     ngrid = int((theta1+theta0)/dgrid) + 1