this makes it so that the input file can be, but does not have to be, specified on the command line in the -o output/file/path.
makes certain that the end-messge is ok.
single BI call.
extraneous try
Merge branch 'output_file_option' of https://github.com/katyhuff/cyclus into output-fname
fixed an indent and added a comment
fixed bug that was noticed by @gidden in which providing just the filename borked.
added comments and a slight modification to @katyhuff's PR. I explicitly call the command line argument for a possible output path arg to deliniate between the possibility of it being simply a file name or not in order to increase code clarity. this has been tested and runs as expected. note that there is currently no unit tests for this behavior. Katy and I discussed the possibility of adding helper classes to the App's reading of command line arguments and subsequently calling code in the Core. Such classes could employ the aforementioned unit tests.
merging @katyhuff's work with my slight modifications