Support transferring and cloning of agents between models

Turtles:

• ls:move <source-model> <reporter-of-turtle/turtleset> <target-mode> <optional-initialization-block>
• Can transfer/clone (kill in one, create in another) between models with the same turtle/breed variable names. That is, can copy if:
  • Does a breed with the same name as the turtle's breed exist in the target model?
  • Take all the names of the turtle's turtle and breed vars. Do variables with the same names exist in the target model?
  • Does the turtle's shape exist in the target model? (we could just default shape if not)
  • If yes to these questions, can transfer.
• Variables are matched up by name, except for position
• Position is tough since the worlds will have different dimensions, and even if they have the same dimensions, it's unlikely that the user will want the turtle's position to be preserved. Right now, I think the turtle should just appear at 0,0, and there should be an optional command block that it runs upon creation that receives xcor as ?1 and ycor as ?2.

Patches:

• Clone only

Links:

• Not sure

No files, tests, or entry points are named. Start by locating existing model, turtle, breed, and link transfer code and related tests, then clarify the move/clone semantics and unresolved link behavior. Done means the supported transfer commands and compatibility rules are implemented with tests for matching variables, shapes, positions, and initialization blocks.