move constants to common

Constants will be used for multiple model types

But for now to work with this script:

  ./node-bin/convert-mml-files

This move requires a symbolic link in:

  md2d/models/engines
new PTA model-type, four-turtles appear in view

partial list of what works:

PTA: patches, turtles, and agents ...

Though the only feature implemented is a *very* rudimentary
turtle feature so far ...

- new model-type 'pta' integrated w/interactive controller
- the turtles have a direction and velocity and move
- horizontal and vertical model space wrapping
- view container integrated with layout system
- code and model editors
- model serialization
- tick history
- benchmarks
JavaScript mixin experiments