Added 'fm3p', 'fm4p', 'fm2r' and 'fm4r' oscillator units

The 'fmNp' units run the N-1 operators in parallel, modulating the master
operator with the sum of their output.

The 'fmNr' oscillators split the operators in two chains and combines the
output from these chains using ring modulation.

Added "fmtest2.a2s", comparing pure sines generated by 'wtosc' and 'fm1'.

Added "fmtest3.a2s" and "fmtest4.a2s", which demonstrate the new FM units.

Closes #174.