Signals clone method

[k1o0]

Is your feature request related to a problem? Please describe.
A method for cloning signals, namely visual signals. If we can't think of a reason to clone normal signals then this should be made a method of Subscriptable signals only. A couple of users have mentioned wanting this.

Describe the solution you'd like
A way to create a number of visual stimuli at once, using a single stimulus as a template:

stimulusLeft = vis.grating(t);
stimulusLeft.altitude = 10;

[stimulusRight, stimulusMiddle] = stimulusLeft.clone();

[stimulusLeft, stimulusRight, stimulusMiddle] = deal(-90, 0, 90);

Additional context
Bonus points for creating these during an experiment. This would probably be impractical though...