Add ability to select window for prevalence calculations

[gkarthik]

Add optional parameter to cumulative prevalence end points: start_date and end_date.

Can function as follows,

start_date	end_date	computed window
None	None	ndays before current date
Specified	None	ndays after start date
None	Specified	ndays before end_date
Present	Present	start_date to end_date; ndays not used