Create an implicit part for a Voice outside a Staff

[wattdave]

Builds on the \addlyrics fix (#196). This PR is based on that branch, so it also contains that commit; please review #196 first.

Problem

A \new Voice placed directly in a group has an implicit Staff in LilyPond:

\score {
  \new StaffGroup <<
    \new Voice = "S" { \set Staff.instrumentName = #"Cantus" c''2 d'' }
    \addlyrics { Du tout }
    \new Voice = "B" { c2 g, }
  >>
}

ly musicxml exports no parts at all for this. In a real four-voice CPDL score,
where the Voices use note variables and \set Staff.instrumentName, it also
prints 'NoneType' object has no attribute 'name'.

Root cause

In ParseSource.check_context() (ly/musicxml/lymus2musxml.py), the Voice
branch only opens a new section. No part is created unless a Staff context
does it. Because nothing ever adds the section to a part, the music is dropped,
and Mediator.set_partname() writes to self.part, which is None.

Fix

When a Voice context starts with no part open, create one with
mediator.new_part(). When that Voice ends, add its music to the part with
mediator.check_part(), the same way a Staff is handled. A new implicit_staff
stack tracks this for nested Voices.

Tests

• Added tests/test_xml_files/voice_in_group.{ly,xml} and test_voice_in_group.
• Before the fix, the test fails because the output has no parts. After the fix,
  the full suite gives 20 passed and 2 xfailed. The 2 xfails were already there.
• On the full Brumel "Du tout plongiet" CPDL score (convert-ly'd to 2.26), the
  output went from an empty document to 4 parts with notes and lyrics.

Not addressed

• LayoutContext and Keyword warnings: the handlers for \layout/\midi
  blocks are not implemented. These warnings are harmless.
• Measure boundaries are still wrong for this score, also with Fix bar counting and markup placement in MusicXML export #193 merged on top (tested: it merges cleanly, but the output is unchanged). Long notes that cross a barline in 2/2 are not split, so measures come out with the wrong length. That looks like a separate issue.
• Incipits written with \cadenzaOn, and bar checks used where there are no
  barlines, still produce measures of the wrong length.

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