use the formula evaluation in write case for the gun LV to avoid changes when gun HV is ON. If happen raise an exception with a message to the user.

git-svn-id: https://svn.code.sf.net/p/tango-ds/code/DeviceClasses/AcceleratorComponents/LinacAlba/trunk@18770 3b39871e-fe4b-0410-b5db-e82997f68959