additional parameter properties

- custom properties: to be declared at startup calling the
  classmethods Parameter.add_property or Command.add_property
  Probably the call should be placed in secop_<facility>/__init__.py

- valid properties are listed in Parameter.valid_properties /
  Command.valid_properties. New properties without relation to code
  in the SECnode like fmtstr, group, visibility can be added at one
  place in code only

Change-Id: I8c550eba955473665d246ddef3815b23c68be4f7
Reviewed-on: https://forge.frm2.tum.de/review/19611
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>

etc/cryo.cfg

@@ -46,3 +46,6 @@ d.group=pid
 
 value.unit=K
 
+# test custom properties
+value.test=customized value
+