Upgrades to synth and EQU inputs

I renamed SynthEQ1 to SynthEquationPlayer and also created
SynthEquationLooper.  You can now switch between synths using the EQU
knob (which will eventually be renamed "PRG" (for "Program") once the
front panel is updated.  You might notice that the audio output from
SynthSubtractor is very quiet.  I need to talk to Gaetan about the ADSR
values again, which are most likely incorrect in this synth.