changed/fixed behaviour of stop commands in PPMS

- make sure new target created by the stop command is between
  old and new target
- fixed bad behaviour of level reading

Change-Id: I484c0902c694c0edb81e2d9238985c05f92e04f4
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/22100
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/lib/__init__.py

@@ -81,7 +81,7 @@ def clamp(_min, value, _max):
     """return the median of 3 values,
 
     i.e. value if min <= value <= max, else min or max depending on which side
-    value lies outside the [min..max] interval
+    value lies outside the [min..max] interval. This works even when min > max!
     """
     # return median, i.e. clamp the the value between min and max
     return sorted([_min, value, _max])[1]