biblio: handle multiple `<imprint>` in books

[SteffenAstheimer]

There are two books with multiple imprints – as they comprise of multiple volumes. So technically they are three seperate books, each belonging to the same series. Although they share the same title... Not sure how to approach that, but would suggest to split them up.

<biblStruct xmlns="http://www.tei-c.org/ns/1.0" xml:id="A112551" status="candidate" type="book">
<!--
	2: Anmerkungen zur deutschen Richtung [? Dichtung?] I (Exz); zu Danzis Karlsruher Tätigkeit (auch II); Begegnung mit Weber in Dresden (III)
-->
   <monogr>
      <author key="A001025">August Klingemann</author>
      <title level="m">Kunst und Natur. Blätter aus meinem Reisetagebuche</title>
      <imprint>
         <pubPlace>Braunschweig</pubPlace>
         <publisher>G.C.E. Meyer</publisher>
         <date when="1819">1819</date>
         <biblScope unit="vol">1</biblScope>
         <!-- https://archive.org/details/bub_gb_q8w5AAAAMAAJ -->
      </imprint>
      <!--<imprint>
         <pubPlace>Braunschweig</pubPlace>
         <publisher>G.C.E. Meyer</publisher>
         <date when="1823">1823</date>
         <biblScope unit="vol">2</biblScope>
         <!-\- https://archive.org/details/bub_gb_Qc05AAAAMAAJ_2 -\->
      </imprint>
      <imprint>
         <pubPlace>Braunschweig</pubPlace>
         <publisher>G.C.E. Meyer</publisher>
         <date when="1828">1828</date>
         <biblScope unit="vol">3</biblScope>
         <!-\- https://archive.org/details/bub_gb_OoUaAAAAYAAJ -\->
      </imprint>-->
   </monogr>
   <keywords scheme="WeGA_biblio">
      <term>Theater</term>
   </keywords>
</biblStruct>

and

<biblStruct xmlns="http://www.tei-c.org/ns/1.0" xml:id="A112049" status="candidate" type="book">
<!--
	Archiv: Sem. * B (Yp 1218)
-->
   <monogr>
      <author>Oscar Teuber</author>
      <title level="m">Geschichte des Prager Theaters</title>
      <imprint>
         <pubPlace>Prag</pubPlace>
         <publisher>Hasse</publisher>
         <date when="1883">1883</date>
         <biblScope unit="vol">1</biblScope>
         <!-- https://www.digitale-sammlungen.de/de/details/bsb11641206 -->
      </imprint>
      <!--<imprint>
         <pubPlace>Prag</pubPlace>
         <publisher>Hasse</publisher>
         <date when="1885">1885</date>
         <biblScope unit="vol">2</biblScope>
         <!-\- https://www.digitale-sammlungen.de/de/details/bsb11641207 -\->
      </imprint>
      <imprint>
         <pubPlace>Prag</pubPlace>
         <publisher>Hasse</publisher>
         <date when="1888">1888</date>
         <biblScope unit="vol">3</biblScope>
         <!-\- https://www.digitale-sammlungen.de/de/details/bsb11641208 -\->
      </imprint>-->
   </monogr>
   <keywords scheme="WeGA_biblio">
      <term>A130006</term>
      <term>Theater</term>
   </keywords>
</biblStruct>