Add midi velocity function

[Lelant]

• now you can use midivelocity() similar to midigate() and midifreq() like this for example:

midigate(1).ar(0.01,0.2).mul(saw(midifreq(1))).mul(midivelocity(1)).mul(.8).out()

note: fork doesn't work yet with midivelocity(), don't know why.

[Lelant]

I need to test this a bit more first. Sometimes it's not working

[Lelant]

Found out that other midi channels block other channels. like the velocity is not set on other channels if you don't play another channel once first. Then it works. This only affects if the sound is played at all, not the velocity value.