Add change of mensuration in **mens representation

Done:

Test data:

**mens
*clefC3
*met(C|)
[sB
sA]
Sc
*met(C)
SA
sB
Sc
sA
*-

MEI conversion:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-model href="https://music-encoding.org/schema/4.0.0/mei-all.rng" type="application/xml" schematypens="http://relaxng.org/ns/structure/1.0"?>
<?xml-model href="https://music-encoding.org/schema/4.0.0/mei-all.rng" type="application/xml" schematypens="http://purl.oclc.org/dsdl/schematron"?>
<mei xmlns="http://www.music-encoding.org/ns/mei" meiversion="4.0.0">
 <meiHead>
  <fileDesc>
   <titleStmt>
    <title />
   </titleStmt>
   <pubStmt />
  </fileDesc>
  <encodingDesc>
   <appInfo>
    <application isodate="2020-09-03T18:04:29" version="3.0.0-dev-ae6b495">
     <name>Verovio</name>
     <p>Transcoded from Humdrum</p>
    </application>
   </appInfo>
  </encodingDesc>
  <workList>
   <work>
    <title />
   </work>
  </workList>
 </meiHead>
 <music>
  <body>
   <mdiv xml:id="mdiv-0000000066747389">
    <score xml:id="score-0000000105725325">
     <scoreDef xml:id="scoredef-0000001977382886" midi.bpm="400">
      <staffGrp xml:id="staffgrp-0000000430790834">
       <staffDef xml:id="staffdef-0000000426347188" n="1" notationtype="mensural.white" lines="5">
        <clef xml:id="clef-L2F1" shape="C" line="3" />
        <mensur xml:id="mensur-L3F1" prolatio="2" tempus="2" dot="false" sign="C" slash="1" />
       </staffDef>
      </staffGrp>
     </scoreDef>
     <section xml:id="section-L1F1">
      <staff xml:id="staff-0000000891517337" n="1">
       <layer xml:id="layer-L1F1N1" n="1">
        <ligature xml:id="ligature-L4F1" form="recta">
         <note xml:id="note-L4F1" dur="semibrevis" oct="3" pname="b" />
         <note xml:id="note-L5F1" dur="semibrevis" oct="3" pname="a" />
        </ligature>
        <note xml:id="note-L6F1" dur="brevis" oct="4" pname="c" />
        <mensur xml:id="mensur-L7F1" prolatio="2" tempus="2" dot="false" sign="C" />
        <note xml:id="note-L8F1" dur="brevis" oct="3" pname="a" />
        <note xml:id="note-L9F1" dur="semibrevis" oct="3" pname="b" />
        <note xml:id="note-L10F1" dur="brevis" oct="4" pname="c" />
        <note xml:id="note-L11F1" dur="semibrevis" oct="3" pname="a" />
       </layer>
      </staff>
     </section>
    </score>
   </mdiv>
  </body>
 </music>
</mei>

humdrum-tools / verovio-humdrum-viewer

Add change of mensuration in **mens representation #408