output Sub Path
Resolves a path to the named subdirectory under the base output directory WITHOUT creating it. This is the path-only sibling of createSubDirectory.
Model's constructor-default output path uses this so that a model that never writes output materializes no directory. The directory is created later, on first actual output (when the model's output directory is first used).
Output abstractions that DO write at construction time (e.g. the scenario / experiment runners, which open a results KSLDatabase in their output directory) must keep using createSubDirectory so the directory exists.
Return
a path to the named subdirectory under the output root, not created
Parameters
dir Name
the name of the subdirectory to resolve. It must not be null