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tempo: fit the phase to a supplied tempo

--bpm replaced the tempo the estimator had chosen, but only after the estimator
had already fitted a phase to that other tempo. The two then described different
grids. Fitting a phase across a whole song at the wrong period lands it at the
song's midpoint, so the offset came out half the accumulated drift wrong, and
the reported figure was a tempo and an offset that had never been computed
together.

On the song this was found with, the search snaps 128.199 to 128.000 and the
offset it printed was the phase at 128.000. Asked for 128.199 the estimator now
answers -0.1668, which is where folding puts the beat at that tempo and within
nine milliseconds of the value the author had settled on by ear.

The tempo now reaches the estimator, which reports a supplied one back with a
confidence of zero: nothing was weighed against anything, so there is no ratio
to give. A supplied tempo also survives an envelope too short or too quiet to
find a phase in, rather than being discarded along with the failed fit.

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

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