Generator wavetables about sfizz

[jpcima]

EDIT changed size*2+1, a benign mistake I copied everywhere, to size/2+1
also made the triangle wave start upwards instead of downwards

About this I don't know, didn't check against real oscillator, but I remember the LFO triangle was starting going up.

EDIT checked against ARIA, all waves starting upwards, fixed the square also

[jpcima]

It's implemented in the PR #61
For a short summary of how this thing works:

• there exists a class HarmonicProfile which is a spectrum generator for any kind of periodic wave.
  You invoke this with successive indices (DFT bin), this returns the amplitude and phase as complex number. It starts at index 1 for fundamental f0, then 2*f0, 3*f0, up to any size.
  The spectrums of conventional waves extend to infinity, that we are able to truncate at whichever point we want.

• it can generate into an array of any size, by filling a spectrum of the matching size and doing IFFT.
  When parameter cutoff is set at 0.5, the whole harmonic range is generated up to Nyquist, or <0.5 to cut it sooner.

• there exist multisamples, which wrap this to generate sets of wavetables for different frequency ranges (currently 1 per octave). For each table, this adjusts cutoff such that, when the table is repitched to the range's highest frequency, the scaled harmonics do not exceed Nyquist frequency.

• one can plot a multisamples with sfizz_test_wavetables and option -m. Pictured below.
  When all is well, the final range of 10-20kHz reduces the signal to a single sine wave.
  

• the demo program permits to listen to an oscillator generated in realtime with a frequency sweep.
  When frequency modulation causes a jump from a wavetable to another, this creates a discontinuity, but I don't find it very perceptible.
  A thing of note: in Cakewalk's oscillators, there exists a quality setting called Dual-High, which permits to interpolate from 2 adjacent wavetables of a multisample, and avoids discontinuity created by such jumps.

• regarding custom oscillators from sound files, it's not too hard to modify the system to add it.

Example: sample=period.wav oscillator=on

The process is like this:
- receive a sound file as a float span, compute its FFT
(this FFT will likely needs scaling ; divide by the signal size)
- create an instance of a subclass of HarmonicProfile, that will return each component for indices in bounds of the FFT array just computed, otherwise 0
- invoke the method createForHarmonicProfile, like for any other profile of the predefined waves

[paulfd]

Merged in #61