ppms driver now uses command handlers

this is a simplicifcation for the ppms driver

- the derivation of a changecmd from a querycmd is moved to
  CmdHandler.__init__
- the special treatment of handlers when writing configured
  parameters has moved to CmdHandler.write
- introduced Drivable.isDriving, changed Module.isBusy

Change-Id: I8862ecda9c8cc998bb018bd960f31c9488146707
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/22033
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_commandhandler.py

@@ -96,6 +96,12 @@ def test_CmdHandler():
         CMDARGS = ['channel', 'loop']
         CMDSEPARATOR ='|'
 
+        def __init__(self, name, querycmd, replyfmt):
+            changecmd = querycmd.replace('?', ' ')
+            if not querycmd.endswith('?'):
+                changecmd += ','
+            super().__init__(name, querycmd, replyfmt, changecmd)
+
     group1 = Hdl('group1', 'SIMPLE?', '%g')
     group2 = Hdl('group2', 'CMD?%(channel)d', '%g,%s,%d')