Processing Time Loader and Tests Update

Updates:
1.  load_agency_contacts now loads the latest stats for each agency and office. 
2.  Added a test for new function, get_latest_stats
3. Moved the test for add_request_time_statistics to test_loading 

Issues:
Sometimes *simple* processing times numbers are greater than *complex* processing times. The source of this issue is two-fold:
a. Some inconsistencies originate on foia.gov
b. Pulling the latest stats for each agency and each type of stat may result in collecting stats from different years, which might not be consistent. ex. National Geo-Spacial Agency. 

**Should we serve stats when simple processing time is greater than complex processing time?**
Do you mean the string 'none' or the Python value None?