Cleanup beam scoring code. (issue4001046)

[hanwenn]

Reviewers: wl_gnu.org, reinhold_kainhofer.com, mike_apollinemike.com,

Message:
Hi,

this is a first step in making the beam scoring a bit saner and more
efficient.

Please review; i've been brainwashed by google's coding standards so
there are likely some style issues.



Description:
Cleanup beam scoring code.

Introduce Beam_scoring_problem to encapsulate parameters and methods
for beam scoring.

This is a preparation for using a priority queue on the to-be-computed
scores.

Document Interval::delta().


Don't crash on a pending spanner in Staff_symbol_engraver dtor.

Please review this at http://codereview.appspot.com/4001046/

Affected files:
  M flower/include/interval.hh
  M lily/beam-quanting.cc
  M lily/beam.cc
  A lily/include/beam-scoring-problem.hh
  M lily/include/beam.hh
  M lily/staff-symbol-engraver.cc