Consolidate parameter names for annotation specifications

[johann-petrak]

Currently several different names are used for this. Maybe just use annspec everywhere?

To consider: should we distinguish in the name between specs which create a subset of a single set and others which create sets from several sets? Probably not.

Document what exactly an annspec is, continue to support the current way (list of annotation set names or annsetname/types tuples) but in the future, allow instances of annspec objects or lists of annspec objects instead (xpath-like specification).

[johann-petrak]

Parameter "annsets" for Document.show and related functions has been renamed to "annspec"

[johann-petrak]

Also changed in the gate worker and pampac modules.