tweak AnnotatedSentence wrapping functions to behave more reasonably when passed a sentence tat already is an AnnotatedSentence

add two new writers: a dummy "none" writer that disables output and a "stats" writer that outputs general info only about token counts and annotation info

Reader factory. There is some code duplication with the writer factory as well as among the individual Readers, but overall it is now easier to support more input formats.

Add Tagset::parse_symbol_string for convenience parsing of comma-separated symbol strings

Fast and drity (regex-based) XCES parser based on the one in Pantera, noticeably faster but prone to breakage on even slightly freeform xml files

update AnnotationChannel interface, fix make_iob_from_segments bug, upgrade make_annotation_vector, annotation heads