bug fixes in ppms

- bug fixes
- set min/max datatype property of mf

Change-Id: I841e69b59085991e56499efbbf9bee7040dc8b59
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21483
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

cfg/ppms.cfg

@@ -13,6 +13,8 @@ class = secop_psi.ppms.Temp
 
 [module mf]
 class = secop_psi.ppms.Field
+target.min = -9
+target.max = 9
 .description = magnetic field
 .iodev = ppms