Fix metworkflow 2

final fixes to make workflow run all the way to create sipnet file

@efblack2 check out modules/data.atmosphere/inst/scripts/met.workflow.R for a simple workflow
To work with convert.inputs, all the met functions have to start with the arguments of in.path, in.prefix, and outfolder, with any optional arguments coming after that. Therefore, I think we should move the start_year and end_year arguments to after those three (and make sure all other met functions also follow that pattern). Also, it looks like start_year and end_year here are integers of just the year, but in issue #272 we definitely ruled that out as the standard (though there never was a final decision between datetime and posix)