ProcedureDefinition holds a FrontEndProcedure instead of a StructureDeclarations.Procedure
Add agentClassString to core Instruction
Consolidate syntax and agentClassString methods into FrontEndProcedure
Convert blockAgentClassString from String/null to Option[String]
Move AgentTypeChecker from MiddleEnd to FrontEnd

- _call and _callreport hold FrontEndProcedures, to simplify the
  type-checking process
Bump sample-scala sbt version
Task variables checked as part of compiler front end.

- _task is decomposed into _reportertask and _commandtask before leaving
  the compiler front end
Verify let in parse before sending to compiler backend

- Rename error from LocalsVisitor.notDefined to LetVariable.notDefined
_let expects only one argument - its initialization value
Merge pull request #27 from NetLogo/wip-mitosis-agent-typecheck-sooner

Wip mitosis agent typecheck sooner