Allow user to specify R0 (in lieu of doubling time)

Calculation (extends "more info about this tool"):

We can compute the implicit $R_0$ from our doubling time inputs and our assumed duration of infectiousness $(1/\gamma)$. Since $R_0$ is the number of additional individuals infected by each infected individual when $I << S$, we can compute the growth rate $g$ as a function of $R_0$ and $\gamma$:
$$g = \gamma R_0 $$
From our previous equation for $g$, we can substitute and rearrange to find:
$$R_0 = (2^{1/T_d} - 1)/\gamma$$

So, for our $\gamma$ of 1/14 days, a doubling time of 4 days corresponds to an $R_0$ of 2.65.

Start by locating the existing doubling-time input and calculation described in the issue. Determine where an R0 input would replace or complement that input, then verify that the model produces the stated relationship for gamma of 1/14 days and a four-day doubling time. Done means the supported input choice and resulting calculation are clear and validated.

Written by the indexing model from the issue text.