Add GainDB ugen

[nshaheed]

No more linear scaling!

// set a Gain with a deciBel value now
SinOsc s => GainDB g(-6.0) => dac;

<<< g.gain(), Std.dbtolin(g.gain()) >>>;

3::second => now;

outputs:

-6.000000 0.501187

[nshaheed]

an alternative implementation of this could be a Gain.db function, i.e.:

SinOsc s => Gain g => dac; // use a gain as normal

-6 => g.db; // set inside gain
<<< g.db() >>>; // prints out "-6"

However, this means that constructors always have to be in linear scaling.

[nshaheed]

@gewang as discussed in pr-lab I've made the following changes:

• GainDB now inheirits from Gain, the only difference between the two is that the GainDB constructor uses dB scaling rather than linear scaling
• Added Gain.db getters and setters so that any Gain object can bet set with db values