I changed the watchdog system for PRM so that the outputs will decrease gradually when the watchdogs are tripped.

** Motivation **

When the watchdogs trip, the outputs are all cut off to 0 suddenly.
Since some outputs, e.g. GAS control, have DC offset, this sudden shutdown makes the large shocks in suspension and causes the mirror slip.
Therefore, we want to modify the watchdog system so that outputs are decreased to 0 gradually.

** What I did **

The new C function ModerateWD.c is used for this purpose.
The function generate the flag that is gradually raised up to 1 according to the combined WD status.
1-(flag) is multiplied to all the outputs to shutdown.
When any of the watchdogs of stages in the suspension trips, all the output is shutdown.
After the all outputs are shut down completely, DACKILL is triggered.

The ramp time to shutdown the outputs can be changed from K1:VIS-PRM_SHUTDOWN_RAMPT. Note that 5 sec is default.

It seems to be working so far. I will implement it in all of PRs as a next step.