Fix a typo.
Implemented basic pinch functionality.

For now it just works for one pinch at a time.
Naive first stab at removing pinches.
Add a bit of documentation.
Keep track of the active PC segment
Do some bookkeeping to keep track of which segments are strummed.

This also introduced "strum*" as proposed by @syntagmatic.
Make sure that the second point is never exactly on the axis.

If allowed to be exactly on the axis, it will not select anything.

@syntagmatic do you understand why (and if so car to explain)?
First stab at getting filtering working with multiple strums.