Improve print message for unexpected tags

The message that is printed for unexpected tags was improved. It now
includes the class of the element (if there is a class). This was done
to make the error reports for inspected folia files more clear.
Update script to download pdf file for play

The generate_kaf script was updated to download the play's pdf file from
dbnl.nl and save it in the dir with the kaf-files.
Update generate_kaf (disable checking of folia files)

The generate_kaf script was updated not to check folia files before
generating kaf files, because only one of the selected texts was valid
according to the inspect_folia script. And upon inspection of the error
reports, the problems with the folia files were only minor. Therefore it
was decided to generate the kaf files without checking the files first.
Add act2kaf function

Some folia files can't be parsed using BeautifulSoup or standard
lxml.parse(), probably because of memory issues. Therefore the folia2kaf
script will be refactored to use lxml.iterparse(). In preparation for
this change, the code that generates a kaf file for an act was moved to
a separate function. The idea is to parse the entire document with
iterparse, extract the xml for an act, load that part into memory,
and than use the existing functionality to generate a kaf file.
Update folia2kaf script to use iterparse

Because some folia files could not be loaded into memory, the folia2kaf
script was updated to use lxml iterparse instead of 'normal' parsing.
For normal parsing, complete xml files are loaded into memory.
The new folia2kaf script only loads the relevant parts of the folia
files into memory (i.e., the acts of theatre texts). Now all folia files
of the 33 plays selected for annotation can be transformed to kaf
without problems.