use a higher order polynomial for sawtooth DPW

[philburk]

Based on a more recent paper by Vesa Välimäki, the SawtoothOscillatorDPW algorithm could be improved.

https://mac.kaist.ac.kr/pubs/ValimakiNamSmithAbel-taslp2010.pdf

• Use a 4th order polynomial.
• Checked the scale factor. May be off by 2.
• Check continuity around VERY_LOW_FREQUENCY.
• Code could be refactored to only calculate diffed inside the conditional.
• Comment styles are inconsistent, "/**/" vs "//".

https://github.com/philburk/jsyn/blob/master/src/main/java/com/jsyn/unitgen/SawtoothOscillatorDPW.java