Enough with multiplicative amp parameters and 0.001

[dpwe]

amp_coefs is unique among the CtlCoefs in that the components are combined by multiplying rather than summed, and as a result default values of zero are ignored, so to silence a voice we must set amp_coef[COEF_CONST] = 0.001. Other codebases are laughing at our ungainliness.

Instead, we should move to log-domain amplitude specification so we can sum instead of multiplying, and 0 means "no change".

The units of amplitude could be Bels (I.e., 10s of deciBels), with 0 meaning inaudible, and a target level of 6 (60 dB, I.e. the same contrast as 0.001 to 1.0 in the current scheme).

We could have the default amp_coef[COEF_VEL] be 6.0 or 8.0 so a normalized key velocity between 0 and 1 exercises the range.