improving the simulator to behalve more like the linac's plcs.
add a general boolean attribute to allow or stop the registers update.
have the readback registers related with the setpoint and its switch attributes.

git-svn-id: https://svn.code.sf.net/p/tango-ds/code/DeviceClasses/Simulators/LinacAlbaSimulator/trunk@18859 3b39871e-fe4b-0410-b5db-e82997f68959