Feature s3 posterior methods

[seabbs]

This PR adds a S3 method workflow and streamlines the lower level workflow. It builds on top of (and fixes issues with #100)

Features

• Adds summary and plot methods for forecast() and fc_tidy_posterior() as interfaces to lower level functions.
• Adds a new summary variable for the transmission advantage of the VOC to the output from fv_tidy_posterior().
• Adds a new plotting function plot_voc_advantage() to extract and plot this.
• Adds plot_voc_advantage() to plot_posterior as an additional output.
• Updates all example data to include this new output.

Breaking changes

• The variant fraction label has been renamed voc_frac from voc due to the new variant advantage label.
• plot_voc() has been renamed plot_voc_frac() due to the new plot_voc_advantage() function.

Bug fixes

• Fixed an issue where voc_label was not being passed by forecast() to lower level methods and hence new labels specifying the VOC were not being assigned.

[codecov[bot]]

Codecov Report

Merging #101 (f2eaa27) into main (01d9e57) will decrease coverage by 7.82%. The diff coverage is 42.63%.

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
- Coverage   95.19%   87.36%   -7.83%     
==========================================
  Files          11       13       +2     
  Lines         707      815     +108     
==========================================
+ Hits          673      712      +39     
- Misses         34      103      +69     

Impacted Files	Coverage Δ
R/check.R	85.00% <ø> (ø)
R/data-scenarios.R	100.00% <ø> (ø)
R/forecast-methods.R	0.00% <0.00%> (ø)
R/posterior-methods.R	0.00% <0.00%> (ø)
R/utils.R	75.00% <22.22%> (-25.00%)	:arrow_down:
R/postprocess.R	98.63% <95.83%> (+0.08%)	:arrow_up:
R/forecast-tools.R	100.00% <100.00%> (ø)
R/forecast.R	92.85% <100.00%> (+0.43%)	:arrow_up:
R/plot.R	91.48% <100.00%> (+3.42%)	:arrow_up:

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