added --flat-tpdf dither option

added the ability to use flat tpdf dithering (no noise-shaping) by using the --flat-tpdf option (this only works in conjunction with the --dither option; no effect otherwise)

Also, cleaned-up a lot of the code

other recently-added options:
--seed n (new in v1.1.5) sets the seed for the random number generator used for dithering. n is a 32-bit signed integer
--relaxedLPF (new v1.1.4) option uses a more relaxed low-pass filter, with roughly double the transition band of the standard filter. This may theoretically allow a small amount of aliasing to occur (at high frequencies), but has the advantage of less ringing.