Expose More Functions/Models

[IsaccBarker]

We need to expose the following methods from db:

• [x] get_reproductive_number()
• [x] get_transmissions()
• [x] get_generation_time()

Again, let's go for a test where the function calls work. We also need to have examples plotting these quantities:

• For the reproduction number, a scatter plot with time on x and Rt on the ys.
• For transmissions, perhaps only a head, looking into the first few observations.
• Generation time would be the same as the Rt case.