implement get_pos_value_mask and get_pos_required_mask
use int for *_count to avoid signed/unsigned errors
handel missing required attributes partially, add a todo

move count_bits_set to pwrutisl, put lowest_bit alias there, implement Tagset::tag_size and Tagset::tag_is_singular

In case the xces document contains sentences with no chunk containing them, a fake chunk will be created to hold the sentences.
Previously this situation threw an exception.

add disamb-ignoring equality
use both of these in Token's remove duplicate lexemes to mEdukacjaWeb/bilanse.do?event=switchBilanse&bilanse=prognozowaneake disamb take priority in case of a disamb/ndisamb dupe

Libcorpus2 now contains the base strructures (tokens etc), their I/O and related classes.
Libpwrutils (name TBD) contains basic building blocks common to all tagger-related libraries
Tagset-tool only needs libcorpus, so was brough here as well.