Linting overhaul

[seabbs]

This PR is inspired by @Bisaloo recent PR to the {epinowcast} package.

It changes the linting GitHub Action to only trigger on changed files and also updates .lintr to make use of a wider range of linters.

As this then introduces a large number of linting flags it then resolves all of these.

In testing this PR I noticed that there were some issues with the snapshot tests for score() and summarise_score(). I have resolved these.

[codecov]

Codecov Report

Merging #284 (60b55f1) into master (ebdfc6c) will decrease coverage by 0.75%.
The diff coverage is 64.91%.

❗ Current head 60b55f1 differs from pull request most recent head 0efda39. Consider uploading reports for the commit 0efda39 to get more accurate results

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
- Coverage   90.23%   89.48%   -0.75%     
==========================================
  Files          22       22              
  Lines        1331     1351      +20     
==========================================
+ Hits         1201     1209       +8     
- Misses        130      142      +12     

Impacted Files	Coverage Δ
R/correlations.R	75.00% <0.00%> (-18.75%)	⬇️
R/input-check-helpers.R	76.62% <0.00%> (ø)
R/summarise_scores.R	81.39% <20.00%> (-8.35%)	⬇️
R/bias.R	75.34% <100.00%> (ø)
R/check_forecasts.R	87.71% <100.00%> (+0.21%)	⬆️
R/convenience-functions.R	81.57% <100.00%> (ø)
R/pairwise-comparisons.R	87.50% <100.00%> (+0.19%)	⬆️
R/pit.R	77.35% <100.00%> (ø)
R/plot.R	97.54% <100.00%> (+0.02%)	⬆️
R/score_quantile.R	100.00% <100.00%> (ø)
... and 1 more

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[nikosbosse]

Amazing, thanks a lot!