advanced leveler

[trummerschlunk]

(moved to low-prio, as not critical for first release)

• feedback circuit
• expander in brake
• scaled speeds for 'above target' and 'below target'
• deadzone (strong break if feedback signal is close to target)
• investigate 'rubberband effect' at higher 'length' for lk2_var

[magnetophon]

To clarify this issue a bit: I already wrote all of these, in this branch and it works wonderfully on my computer, but for some reason @trummerschlunk is seeing a weird bug with that branch:
The gain jumps randomly sometimes.

The WIP branch where I originally experimented and came up with these features, does not have that weird bug, even though it's a slightly more complicated version of the same code and it even misses some fixes for divide by 0 that I have implemented in the advanced_leveler branch.

@trummerschlunk Did I summarize the bugs you are seeing corrctly?

[trummerschlunk]

Yes!