[Patch] Add support for tempo ranges (issue3248042)

[v . villenave]

Reviewers: ,

Message:
Greetings everybody,

here's a patch to add the ability to enter tempo indications as number
ranges:
   \tempo 4 = 76~82

Please have a look and let me know!

[PS: the side-by-side diff doesn't work for me, but please have a look
at the unified diffs!]

Description:
Add support for tempo ranges

This patch set
 - extends the parser to accept the following syntax:
     \tempo numA ~ numB
 - creates a 'tempoUnitRange property (a pair of numbers)
 - sets 'tempoUnitCount accordingly by averaging the two
numbers whenever appropriate
 - modifies the metronome engraver to return the range pair
instead of the number when appropriate
 - adapts the format-metronome-markup translation function
to print the tempo range properly
 - adds a metronome-range.ly regtest.

Please review this at http://codereview.appspot.com/3248042/

Affected files:
  A input/regression/metronome-range.ly
  M lily/metronome-engraver.cc
  M lily/parser.yy
  scm/define-context-properties.scm
  scm/ly-syntax-constructors.scm
  scm/translation-functions.scm