Add harmonic by fret and by ratio to tablatures (issue3326042)

[Carl . D . Sorensen]

Reviewers: marc, p.l.schmidt_gmx.de,

Message:
I've taken Marc's harmonic.ly and moved the scheme functions into
scm/tablature.scm, the music functions into ly/engraver-init.ly, and the
test music into input/regression/tablature-harmonic-functions.ly.

I've also added docstrings for the functions that are called by the
music functions, since they need to be define-public as they are called
from a ly file.

I've removed the parenthesize functions from scm/tablature.scm, since
the parenthesizing is handled in tab-note-head::print.

Please review, and thanks,

Carl


Description:
Add harmonic by fret and by ratio to tablatures

Code written by Marc Hohl  (All the good stuff)
Code integrated by Carl Sorensen (Any mistakes are his responsibility)

Please review this at http://codereview.appspot.com/3326042/

Affected files:
  M ly/engraver-init.ly
  M ly/music-functions-init.ly
  M scm/tablature.scm