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_psi/ppmssim.py

@@ -54,7 +54,7 @@ class PpmsSim:
         self.status = NamedList('t mf ch pos', 1, 1, 1, 1)
         self.st = 0x1111
         self.t = 200
-        self.temp = NamedList('target ramp amode', 295., 1, 0, fast=self.t, delay=10)
+        self.temp = NamedList('target ramp amode', 200., 1, 0, fast=self.t, delay=10)
         self.mf = 100
         self.field = NamedList('target ramp amode pmode', 0, 50, 0, 0)
         self.pos = 0
@@ -75,6 +75,7 @@ class PpmsSim:
         self.start = self.time
         self.mf_start = 0
         self.ch_start = 0
+        self.t_start = 0
         self.changed = set()
 
     def progress(self):