new var onsetDate combines DateOnset and DateReport; onset.last21days true/false
bugs and fixes to period, last 7 days, etc
continued bug fixes and updates
error traps in tabulate.  Added section for tabulating by 7-day periods
initial merge with conflicts onset_date branch

Merge branch 'onset_date'

Conflicts:
	vhf.Rmd
fixed merge conflicts.  Added map and region files.