fix HeadlessModelOpener to forget plots on open()

since initForTesting() creates some
remove unused code
refactor/DRY in headless.lang
relocate tests from headless.model to headless.lang

I'm looking to merge the two packages, which overlap substantially in
functionality, and I'm starting by getting anything out of
headless.model that doesn't actually use ModelCreator
refactoring in headless.model
minor cleanup
cut dependency of TestTreeAgentSet on MockSuite
eliminate headless.model, move the tests to headless.lang

redoing them all in the headless.lang style. bye bye DSL.  (I felt
that having tests written in two styles wasn't worth the benefit,
especially so since the alternate style didn't even share the same
workspace-handling code, so it was redundant and confusing not just
for the person reading and writing tests, but also implementationally)
relocate ModelCreator to headless.lang

since we aren't using it anywhere else
beef up TestPlotModels a bit