Make a piecewise linear interpolating vector of Rt, for each
day within the simulated interval. The assumed start of simulation
is the first date provided in dates, and the end of simulation is the last
date of dates unless max_date is provided. Interpolation is done with
inclusive endpoints.
interpolate_rt(dates, rt, max_date = NULL)a vector of Dates
a vector of reproductive values at those dates
the maximum date of simulation (optional)
a list with named elements Rt and Rt_tt, which can be provided
to the get_parameters function's arguments R0 and tt_R0.