class library: allow NamedControl.new() to return non-arrayed Lag (fixing issue 973)

See [this thread](http://new-supercollider-mailing-lists-forums-use-these.2681727.n2.nabble.com/Bug-with-Control-lag-and-Pan2-td7603440.html) for more details.  Basically, when using NamedControls with a single lag value in a SynthDef, the Control unexpectedly returns an array.

We should maybe wait to see if anyone on the list has any objections, or reasons why this fix could possibly be a bad idea.  I'm not an expert in this area by any means!  But I think it makes sense -- if the input is an Array of lag times, then an Array of Lags is returned.  If it's a single lag time, then a (non-array) Lag is returned.  Similar to what happens with the `value` argument for NamedControl.new().