First CRAN version

The main features of the package include:

• get_R: a function to estimate R as well as the force of infection over
  time, from incidence data; output is an object of class earlyR

• sample_R: a function to obtain a sample of likely R values

• plot: a function to visualise earlyR objects (R or force of infection).

• points: a function using earlyR objects to add the force of infection
  to an existing plot.