bugfix for task 21.
change the message for gun_LV_onc write exception (task 15).
Add the filament voltage to the attributes with the ramping feature (under tasks 8 and 16).

git-svn-id: https://svn.code.sf.net/p/tango-ds/code/DeviceClasses/AcceleratorComponents/LinacAlba/trunk@18856 3b39871e-fe4b-0410-b5db-e82997f68959
another iteration for the ramping feature.
improve documentation on dynattr builder methods.

git-svn-id: https://svn.code.sf.net/p/tango-ds/code/DeviceClasses/AcceleratorComponents/LinacAlba/trunk@18857 3b39871e-fe4b-0410-b5db-e82997f68959