Tempo ramps - remove the concept of bars from tempo sections.

Tempo ramps - remove the concept of bars from tempo sections.

	- this helps where tempo and meter have a somewhat circular
	  dependency.

MetricSection now has a musical position expressed in beats (a double).
MeterSection still has a bbt, but it really isn't needed as we have
enough information to discover the number of bars at a given beat without it.
TempoSection now has a hack to enable loading of legacy sessions, which will
ultimately be a lot cleaner than the current code.

Removing bars from tempo sections also allows us to place them
at arbitrary frames (implemented here).