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warp: fit the tempo segments a song needs, and refuse the ones it does not

drift reported only the single worst stretch, which is no answer for a song that
moves more than once. It now prints every segment needed to hold the grid within
the tolerance, as a command line to paste back, and generate accepts --warp with
no value to fit and apply the same set itself. --warp-slip sets how far the grid
may wander first.

The segments are chosen by dynamic programming over the phase track rather than
by extending pieces left to right. Greedy fitting runs a piece past the change
before the tolerance notices, and the next piece then straddles the change and
reports a tempo belonging to neither side; on a click track stepping from 128 to
130 BPM it invents that bridge.

Two rules then throw boundaries away, because a tempo segment can only say the
beat spacing changed and that is the wrong instrument for a step in the measured
phase. Steps are common: where the beat appears to sit depends on what is
playing, so percussion entering moves the measurement without moving the beat. A
click track at a mathematically exact tempo with off-beat hits entering half way
through produced a spurious excursion of 0.8 BPM at exactly that moment. A
boundary now has to move the grid by more than the tolerance across the shorter
stretch it separates, and a tempo that departs from its neighbour and comes
straight back is treated as a step, since real tempo changes do not undo
themselves. Joined segments take the middle tempo of their pieces by duration,
because a line refitted across a step comes out tilted and reintroduces the
error that was just removed.

Feinak fits four segments where the greedy version wanted nine.

Signed-off-by: Andrew Udvare <audvare@gmail.com>

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