Add value parser + use it for the gui

replaces eval which is used so far

Change-Id: Ie5ff8c82175786e233d52bc0faac4e72e3bc27e9
Reviewed-on: https://forge.frm2.tum.de/review/17271
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>

secop/errors.py

@@ -37,6 +37,10 @@ class ProgrammingError(SECoPServerError):
     pass
 
 
+class ParsingError(SECoPServerError):
+    pass
+
+
 # to be exported for remote operation
 class SECoPError(SECoPServerError):
     pass