Small fix in sorting tei:biblStruct

[TomazErjavec]

I tried to generate HTML for an edition, but the conversion fails with

Type error at char 10 in expression in xsl:sort/@select on line 821 column 65 of html_core.xsl:
  XTTE1020  A sequence of more than one item is not allowed as the @select attribute of
  xsl:sort (<date>, <date>)

the error refers to the line

Stylesheets/html/html_core.xsl

Line 821 in a74859c

<xsl:sort select="tei:monogr/tei:imprint/tei:date"/>

It turns out that some of our imprint elements have more than one date, which is allowed by the standard TEI schema, and also makes sense here: the imprint describes a biblio item that was published in parts in several years. To remove the error, I suggest that the biblStructs are sorted by the first date found in imprint, i.e. change the line to read

<xsl:sort select="tei:monogr/tei:imprint/tei:date[1]"/>

The same thing would have to be done in latex/latex_core.xsl.