Stop using write_csv and write_csv_header during optimize.
Merge branch 'develop' into feature/issue-532-remove_write_csv
added new functions to stan/gm/command.hpp; write_iteration_csv, write_iteration; both unit tested
changing command.hpp to use the write_iteration function
changing command.hpp to use the write_iteration function
Merge branch 'develop' into feature/issue-532-remove_write_csv
adding optimization output test. fixed newton method so it actually runs
removing last remaining call to write_csv
Merge pull request #536 from stan-dev/feature/issue-532-remove_write_csv

Fixes #532. Stop using write_csv and write_csv_header during optimize.