Referring to template parameters in the text of the transmissibility report

[CarmenTamayo]

On the text of the transmissibility report, which is meant to be rendered and read by external people, who haven't necessarily been involved in developing the reports, there are often references to the parameters that are listed at the start of the template. For instance, in the EpiNow2 chunk, it currently says "a table summarising these results for the last r params$r_estim_window days"
However the external readers that haven't been involved in creating the report and are unfamiliarised with the package will have no idea what this is referring to.
For this reason I propose changing these to describing what those parameters mean and, when relevant, mentioning them inside the code chunks instead, as well as in the documentation of the package.
This will be addressed in the branch "tx_pipeline_update"

[CarmenTamayo]

This has been addressed in #144