Convert tempo-based scores only to time-based score. Restructure change and conversion classes.
Handle Gradual::Trimmed changes in ScoreCollator
Refactor ValueComputer using separate Function classes and #to_function methods in change classes
Refactor ValueComputer by adding Function and Transition classes, and #to_transition methods in change classes
Add optional start_value for changes, to make them absolute (non-relative)
Roll version to 0.3.0