[hv.filter peak] misses the gain inlet

[grrrr]

While most [hv.filter] variants have two parameters, the peak variant has three, namely the additional gain level. Since it would be desirable to set the gain by message, a third inlet is needed.
This would also imply that all the filter abstractions in the filter/ folder get a third inlet (if needed or not) to get rid of a "inlet->xxx connection failed" error message.
I would be willing to submit a pull request for this issue.