switch to rounded duration to get i_stop

Change comment 
Add more to strict assertion 
Add comment to explain duration calculation

Author: zm711

neo/io/proxyobjects.py

@@ -187,21 +187,25 @@ def _time_slice_indices(self, time_slice, strict_slicing=True):
                     assert self.t_start <= t_start <= self.t_stop, 't_start is outside'
                 else:
                     t_start = max(t_start, self.t_start)
-                # the i_start is necessary ceil
+                # the i_start is rounded to the nearest sample
                 i_start = np.rint((t_start - self.t_start).magnitude * sr.magnitude).astype(np.int64)
-                # this needed to get the real t_start of the first sample
-                # because do not necessary match what is demanded
+                # this is needed to get the real t_start of the first sample
+                # because it does not necessary match what is demanded
                 sig_t_start = self.t_start + i_start / sr
 
             if t_stop is None:
                 i_stop = None
             else:
                 t_stop = ensure_second(t_stop)
                 if strict_slicing:
-                    assert self.t_start <= t_stop <= self.t_stop, 't_stop is outside'
+                    assert self.t_start <= t_stop <= self.t_stop, 't_stop is outside possible time range'
                 else:
                     t_stop = min(t_stop, self.t_stop)
-                i_stop = np.rint((t_stop - self.t_start).magnitude * sr.magnitude).astype(np.int64)
+                # calculate duration demanded then round it to nearest sample number
+                # add this to i_start to get i_stop
+                delta = (t_stop - t_start) * sr
+                i_stop = i_start + int(np.rint(delta.simplified.magnitude))
+                
         return i_start, i_stop, sig_t_start
 
     def load(self, time_slice=None, strict_slicing=True,