Code review for study definition, data preparation and plotting vaccination dates

[elsie-h]

@wjchulme please could you review the main branch of this repo - if you want to suggest changes please create a branch.

Apologies for doing this as an issue rather than PR tracking changes compared to the original 2dose waning project - I was initially doing this but it got a bit messy with file name changes etc so I thought it better to merge and summarise the changes here.

A summary of changes:

• analysis/dummy_data_vax.R updated with more realistic vaccination dates here
• analysis/study_definition_vax.py unchanged from original project, but with imd updated and some variables removed as they can be defined in second study definition
• analysis/preprocess/input_vax.R preprocessing unchanged, new function my_skim for summarising datasets
• analysis/preprocess/data_eligible_a.R application of eligibility criteria a (see below), criteria b removed as will now be moved later
• analysis/eda/plot_vax_dates.R plot first, second, and third vaccination dates within groups

Eligibility criteria a:

[wjchulme]

Runs, and looks good. I might steal some of those utility functions

[elsie-h]

Thanks! FYI I tidied up the scripts in the ggplot-extensions folder, as I was defining several ggplot2 internal functions unnecessarily (closed PR here).