- after reports that a tempo with non-4.0 note type
would produce a ramp in the preceding section, an incorrect
assumption was made leading to a bogus 'fix' that redefined
Tempo's pulses_per_minute().
the real fix was simple:
the ramp's function constant was calculated using the note type
of the proceding tempo rather than the current one.
- this patch reverts e82482e8e9 and a4d67279e0
and maintains the old definition of Tempo::pulses_per_minute()
74a604d261