Create output control report

[matthiasgomolka]

Think about how to create a report that's useful for both users and output checkers which

• lists all conducted output checks and their results
• contains some metadata on the run, such as timestamp, user running sdc_log()
• ...

Relates to #71.

If #71 was implemented, it would be possible to include the paths to the output files.

[matthiasgomolka]

Maybe initialize the report by creating an instance of an R6 class (e.g. sdc_report), in which output results and checks are stored? Then finalize the report (using a method of the R6 object sdc_report$close) and write to disk?

Re-read https://adv-r.hadley.nz/r6.html before actually doing this.