Improve Ivantsov filters niceness

[p0nce]

• add that their coefficient cannot be smoothed, according to Ivantsov
• Test the nextBuffer functions
• Proper DDoc
• Advertise on Discord and document in Changelog
• Fix the gain situation => later, it's an easy PERF win
• Have defaults for damping, cutoff, type and gain (eg: a 1P shelf without gain), so that you're not force to call them

[p0nce]

Usage is fairly easy.

unittest
{
    IvantsovIIR lp;

    lp.initialize(44100)
      .type(IvantsovType.lowPass1stOrder)
      .cutoff(1000)
      .damping(0.707)
      .gain(0);

    static immutable double[16] input = [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
    double[16] output;

    import core.stdc.stdio;
    lp.nextBuffer(input.ptr, output.ptr, 16);
    for (int n = 0; n < 16; ++n)
    {
        assert(!isnan(output[n]));
    }
}

[p0nce]

A bit of polish for later on:

• DC initialization (to smooth control signal, why not. Fast control-signal might benefit from decramping?)
• remove excess field PERF
• function to get Complex response
• bypass subtype
• SIMD if needed

First working version is v16.4.2