Add support for neumes encoded line by line

[lpugin]

This PR add support for neume notation encoded line by line, with each line encoded in a section. It is part of some refactoring for making the use of facsimile more generic than it now is in the Neon editor. The changes are essentially adjustments backward compatible with the Neon editor where the MEI loaded is cast off in distinct <staff> elements pointing to a facsimile zone. Staves are now expected to be wrapped into section elements with a @type attribute. It breaks the rule that Verovio should not rely on @type, but no other reasonably simple solution could be found.

The encoding structure expected is a follow:

<section>
  <pb/>
  <sb/>
  <section type="neon-neume-line">
    <staff n="1">
      <!-- content of the first line -->
    </staff>
  </section>
  <sb/>
  <section type="neon-neume-line">
    <staff n="1">
      <!-- content of the second line -->
    </staff>
  </section>
  <sb/>
  <section type="neon-neume-line">
    <staff n="1">
      <!-- content of the third line -->
    </staff>
  </section>
</section>

Internally, each section is converted into a pseudo Measure. This is identical to what is happening internally when mensural notation is cast off in Verovio, except the the cast off is actually pre-made with neume lines.

The PR also adds support for staff rotation when the layout is based on facsimile zones.

See examples in the PR to the DDMAL repository