Provide a way of querying the "current" output value of a Signal

[colinbdclark]

There are lots of situations where one might like to get the value of a Signal for purposes other than generating audio, outside of the main sample generation loop. For example, in Starloop (the Bluemchen Looper built with Starlings), the playback speed of a loop is determined by adding two different sources together (using star_sig_Add:

1. a value set with the encoder (using star_sig_Accumulate)
2. the CV signal from the "speedMod" CV input jack

As it stands now, there's now way to "safely" see what value is actually being calculated for the speed. As in Flocking, there should be some means by why the Signal stores its last output (so at control rate) in a conventional place where it can eventually be accessed by the Starlings "bus."