updated imports

- now pulling constants and classes from .gfx rather than .grobs
updated to use plotdevice.lib.register

- register() returns an initial _ctx value (from whatever script
imported it) and signs the module up to have its _ctx updated whenever
a new context is bound
- need to test whether submodules need to register independently or if
they can safely peek at the parent module’s _ctx value (as wordnet,
web, graph, and grid seem to be doing)…
- (it wouldn’t surprise me if they get a stale _ctx after the initial
import)