Make \score block optional

[th-we]

When the data is intended to be used by a "driver" LilyPond file, don't write the \score block and instead write a common variable like this:

common = {
  \key c \major
  time 9/8
}

[th-we]

This can be done by introducing a new <xsl:param name="outputScoreBlock"> at the top level template with

<xsl:template match="mei:scoreDef[$outputScoreBlock=false()]" mode="score-setup">

and create the \common block you see there. For the key and time signature, call the setKey and and meterSig templates.

Right now, the meterSig template needs to be given explicit parameters when called from the context of a <scoreDef> element, like here.

To implement this, create a new feature branch and push that branch instead of master to GitHub. I'll review it then.

Any more info/support needed?