Add support for tempo ranges

Add support for tempo ranges

This commit
 - extends the parser to accept the following syntax:
     \tempo numA ~ numB
 - changes the 'tempoUnitCount property to accept either
a number or a pair (in which case tempoWholesPerMinutes
will be set by averaging the two numbers)
 - adapts the format-metronome-markup translation function
to print the tempo range properly
 - adds a metronome-range.ly regtest
 - adds relevant display-lily methods, and updates this
regtest as well.