Add output_dir argument to rfa_simulate()

[danmcgraw94]

rfa_simulate() currently writes its rfaR_results/ output to the working directory, which can surprise users who don't know where getwd() points (especially first-time R users without an RStudio project).

Scope

Add an output_dir = NULL argument that defaults to getwd() when NULL but lets users specify an explicit path. Document the default behavior clearly.

Feedback source

Raised by Allen (comment #2).