Incorporate 899 into tcl, minor improvements

Add the necessary code to include the 899 laser into TCL for the edm
experiment, also include a function to scan the cavity locking voltage so
that the HeNe peak can be centred again after a crash, and make TCL take
its voltage limits from those initially defined when the analogue output
channels are created (previously these had to be specified sepeartely and
could not differ between lasers)