allow `<unclear>` in general (not only for manuscripts)

[cthomasdta]

I am in favour of this, which is a minor change and a logical consequence from the attribute values used in <supplied> and <unclear>. Any objections?

Originally posted by @cthomasdta in #74

cf. #16 (comment) : atm <unclear> is only allowed in the DTABf for Manuscripts; while I understand the former reasons not to use it for the DTA print/core corpus, I would strongly suggest to allow <unclear> for the general DTABf as well.