Beams not exported in MusicXML + support for multiple beams

[leleogere]

Beams are not exported in MusicXML (although they are imported).

XML before partitura import:

      <note>
        <pitch>
          <step>G</step>
          <octave>4</octave>
          </pitch>
        <duration>1</duration>
        <voice>1</voice>
        <type>32nd</type>
        <stem>up</stem>
        <staff>1</staff>
        <beam number="1">continue</beam>
        <beam number="2">begin</beam>
        <beam number="3">begin</beam>
        </note>

XML after partitura export:

      <note id="n4">
        <pitch>
          <step>G</step>
          <octave>4</octave>
        </pitch>
        <duration>1</duration>
        <voice>1</voice>
        <stem>up</stem>
        <type>32nd</type>
        <staff>1</staff>
      </note>

Additionally (correct me if I'm wrong), it seems that there is no notion of beam levels. For example, an eighth note followed by two sixteenth notes should have a first beam over all of them, and a second one over the last two notes. Currently, a single beam object is created, containing all notes without hierarchy.