Improve Database Methods Coverage

[olivia-banks]

Relevant Codecov:

• https://app.codecov.io/gh/UofUEpiBio/epiworldR/blob/main/R%2Fdata.R

To Bind:

• repnum (row independent)
• plot_reproductive_number
• as.array.epiworld_hist_transition
• get_transmissions.epiworld_diffnet
• get_generation_time

I'm assuming we don't care about unit testing functions that just plot things directly.

Concordant with #77.