title: How to transpose ChordSymbols along with notes in Music21?
body: <p>I'm trying to transpose a jazz lead sheet. When I transpose the Music21 stream, only the notes are being transposed. The ChordSymbols stay the same.</p>
  
  <p>Here's an example in tinynotation of what I'm seeing. Note: I'm not using TinyNotation in my actual use-case, but it works for providing an example here.</p>
  
  <pre><code>from music21 import *
  
  class HarmonyModifier(tinyNotation.Modifier):
      def postParse(self, n):
          cs = harmony.ChordSymbol(n.pitch.name + self.modifierData)
          self.parent().stream.append(cs)
          return n
  
  tnc = tinyNotation.Converter()
  tnc.modifierUnderscore = HarmonyModifier
  tnc.load("4/4 d2_m7 g4_7 c_maj7")
  
  s = tnc.parse().stream
  s.show("text")
  s.transpose(interval.GenericInterval(2), inPlace=True)
  s.show("text")
  </code></pre>
  
  <p>And here's the output.</p>
  
  <pre><code>{0.0} &lt;music21.stream.Measure 1 offset=0.0&gt;
      {0.0} &lt;music21.clef.BassClef&gt;
      {0.0} &lt;music21.meter.TimeSignature 4/4&gt;
      {0.0} &lt;music21.harmony.ChordSymbol Dm7&gt;
      {0.0} &lt;music21.note.Note D&gt;
      {2.0} &lt;music21.harmony.ChordSymbol G7&gt;
      {2.0} &lt;music21.note.Note G&gt;
      {3.0} &lt;music21.harmony.ChordSymbol Cmaj7&gt;
      {3.0} &lt;music21.note.Note C&gt;
      {4.0} &lt;music21.bar.Barline style=final&gt;
  # Note ChordSymbols are not changed after transpose()
  {0.0} &lt;music21.stream.Measure 1 offset=0.0&gt;
      {0.0} &lt;music21.clef.BassClef&gt;
      {0.0} &lt;music21.meter.TimeSignature 4/4&gt;
      {0.0} &lt;music21.harmony.ChordSymbol Dm7&gt;
      {0.0} &lt;music21.note.Note E&gt;
      {2.0} &lt;music21.harmony.ChordSymbol G7&gt;
      {2.0} &lt;music21.note.Note A&gt;
      {3.0} &lt;music21.harmony.ChordSymbol Cmaj7&gt;
      {3.0} &lt;music21.note.Note D&gt;
      {4.0} &lt;music21.bar.Barline style=final&gt;
  </code></pre>
  
  <p>I'm using music21-4.1.0.</p>