Alternative calculation for ```originalSourceNumberOfPages``` (metadata file)

[lehkost]

Back in early 2021, we introduced originalSourceNumberOfPages in #133 to be part of the metadata files. We calculate the page number by subtracting #firstpage from #lastpage (plus one). The information (if available) comes from the <bibl> part where you can specify <biblScope unit="page" from="#firstpage" to="#lastpage">#firstpage–#lastpage</biblScope>.

I propose to calculate the number of pages simply by counting all <pb/> elements (which can be empty or contain a page number, like <pb n="56"/>).

Reason: Some pages have Roman page numbers. And there are plays that were published in journals and have page numbers like "Vol. 1, S. 1–16, Vol. 2, S. 33–45". Calculating the differences would be too complex and by counting the <pb/> elements we would count actual pages.

Of course, page numbers in the bibliographic data can still be tagged as usual, we just wouldn't use them to calculate the number of pages for the metadata files any more.