Breaking change! Temporal args now scale with current bpm

* all args that represent time (i.e. envelope, phase duration) now scale
 with the current bpm. That is, if the bpm is the default, the units
 represent seconds. However, if the bpm is doubled, the times half. This
 means that the units are actually beats. The echo therefore echoes with
 the beat, the envelopes shorten to ensure each note stays within its
 beat remit and the modulation synths modulate in accordance with the beat.
Fix slicer fx to use (/ 1 phase) as rate
update examples to match new arg names and bpm behaviour
Add new fn rt for bypassing any bpm scaling:

+use_bpm 120  # modifies all time to be half
+play 50
+sleep 1      # actually sleeps for half of a second
+play 62
+sleep rt(1)  # bypasses bpm scaling and sleeps for a second
+play 72
allow users to disable bpm scaling for args

* Note that this currently can conflict with use of the rt fn :-(
fix whitespacing
we're heading towards RC12
add start of tests for scale and chords