add default directory for output_folder in step_one

[ryanpeek]

currently if output_folder isn't set by user, step_one_functional_flow_results() will fail with this error:

Error in file(file, ifelse(append, "a", "w")) : 
  cannot open the connection
In addition: Warning message:
In file(file, ifelse(append, "a", "w")) :
  cannot open file 'NA/10017314_ffc_percentiles.csv': No such file or directory

Will add a PR to address this so it defaults to working directory instead of failing, something like:

step_one_functional_flow_results( output_folder=".")