stop reading many frames to get times for shifted_correlation

src/mdevaluate/correlation.py

@@ -147,7 +147,8 @@ def shifted_correlation(
     num_frames = int(len(frames) * window)
     ls = np.logspace(0, np.log10(num_frames + 1), num=points)
     idx = np.unique(np.int_(ls) - 1)
-    t = np.array([frames[i].time for i in idx]) - frames[0].time
+    dt = round(frames[1].time - frames[0].time, 6) # round to avoid bad floats
+    t = idx * dt
 
     result = np.array(
         [