Allow poem title when parent section has no headline
Allow poem title when parent section has no headline

fixes #300

Also added nordic270 which is really redundant but displays a more useful error than the RNG when you try to place a bridgehead in a location other than as a child of body/section/article/div.

mtmse/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator