Feed-forward for fast out when sweeping with analog out

[bleykauf]

EDCLs typically need a fast-forward to the laser current when scanning them via the grating/piezo to avoid mode-hops. Currently this is not implemented in Linien.

Could be done by creating new Module FeedForward that has inputs sweep.y and a user-configurable feedforward factor and outputs the feedforward value that is then added to the fast outputs inside linien.py. I guess that the output should be terminated when the sweep is off to not mess with any existing locking procedures. Simply using sweep.hold should probably do the trick.

I think it is sufficient to only address the case when slow out is used for sweeping and add the factor to the fast output used for control (i.e. the laser diode).