Add schematron to check antilabe markup

[juliajbeine]

@daliboris has written a schematron checking if all antilabes, i. e. verse lines spoken by more than one character, are marked up correctly, which means that <l part="I"> is followed by <l part="M"> or <l part="F">, <l part="M"> is followed by <l part="F">, and <l part="F"> is followed by <l part="I">. (@daliboris please correct me here if I forgot something).

@cmil, could we add this check to the DraCor schema crediting @daliboris?
@lehkost FYI.

[daliboris]

The schema also checks whether numbers are assigned to the lines and whether the lines are numbered sequentially.

[cmil]

@daliboris @juliajbeine The antilabes check is now implemented in #194.

For DraCor plays in general we don't require line numbers. Under what condition should they be required and sequential numbering be enforced? If this needs to be implemented in the DraCor schema, we should open a separate issue.

[juliajbeine]

@cmil, thank you! We do not need the line numbers in DraCor. However, if our sources include them, we keep them, same with page numbers. (For the Roman, Greek, and Neo-Latin corpora, an important reason is that Greek and Latin plays are rather referred to by line numbers than by scenes and acts, as the latter may differ between editions.) Thus, we do not need a line number check in the DraCor schematron.