Implement additional ControlList interpolation methods.

Implement additional ControlList interpolation methods.

The Control and ControlList uses the raw value (eg. coefficient for gain,
Hz for frequencies) and those Lists are stored in existing sessions.

In the vast majority of cases interpolating automation values using exp/log
scale for dB, freq makes more sense -- it's also what the fader does.

Adding additional interpolation methods is future proof (we might at allow
to even add different methods per automation point (to the next) like other
DAWs do.

Currently it's mainly used in preparation for consistent GUI automation-
lanes. Between 2 points there's always a visual straight line.