Added 'speed' setting for ficus' sample playback. This is set using the new function 'ficus_playback_speed()' providing it a bank number and speed float argument. 0.01 thru 0.99 will slow down playback, 1.1+ will speed up playback.  Passing the speed float argument as negative will cuase playback to reverse.  Using 'ficus_playback_speed(bank, 1);' will reset to 'normal' playback settings.