Use a loop to set up all transitions

All of these delay() and attr() calls are identical except for the delay
duration and the header name, both of which we can compute dynamically
from a counter (i.e. the week number) rather than hardcoding them.
Add original version of flu CSV

From https://github.com/rozeggo/1918_d3. This has actual dates as
headers instead of "week_1", "week_2" etc.
Add parseDateHeader function in anticipation of parsing original CSV
Switch to using original CSV with date headers
Remove unused version of CSV
Iterate through dates and headers rather than just the headers

We would ultimately like for the animation to be controlled by the
"current date" (e.g. as provided by a slider), and keeping track of which
header corresponds to which date gets us closer to that goal.
Make sure the dates are sorted before iterating through them

They happen to be in the right order in the CSV anyway, but it doesn't
hurt to make sure.
Extract getDatesWithHeaders() function
Periodically advance date instead of setting up a bunch of timers

This makes the animation "tick" once a second, advancing the current
date on every tick, which will make it easier for us to hook up a slider
later: if you move the slider backwards, the animation will continue
advancing from the new date instead of sticking to the original schedule.
Extract setDate() function

At the moment this function only updates the circles on the map, but
if/when we have other date-related elements (e.g. a date label, a
slider) it can update those too.
Extract headerForDate() function
Extract indexForDate() function
Increment dates instead of date index