Fix #372.

Fix #372.

Thanks to Carl for providing the inspiration for this patch.

* input/regression/auto-beam-partial-grace.ly:

  new regtest

* input/regression/display-lily-tests.ly:

  remove TODO for scaled duration \partial test

* lily/partial-iterator.cc (new file):

  use a simple music iterator to calculate the correct measurePosition
  setting, and warn for \partial used after the start of a score

* scm/define-music-display-methods.scm:

  simplify display method for partial: since the new music object `PartialSet'
  carries the original duration from the parser, extra code for converting
  moments to durations is no longer required

* scm/define-music-properties.scm (all-music-properties):

  add property for \partial, partial-duration

* scm/define-music-types.scm (music-descriptions):

  add PartialSet

* scm/ly-syntax-constructors.scm (partial):

  change constructor to allow 'origin to be set directly (used by iterator
  to signal warning message)

  use PartialSet