Closes make n_strain and n_vacc_classes explicit #353

[RaphaelS1]

@annecori can you please review to check that this is what you had in mind?

@epiyasin as discussed could you do the ncov side? If you don't get a chance next week I'll just do next Fri :)

[codecov]

Codecov Report

Merging #392 (77413d5) into master (bab344f) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 77413d5 differs from pull request most recent head 9f39c25. Consider uploading reports for the commit 9f39c25 to get more accurate results

@@            Coverage Diff             @@
##            master      #392    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           14        14            
  Lines         2922      2676   -246     
==========================================
- Hits          2922      2676   -246     

Impacted Files	Coverage Δ
R/lancelot.R	100.00% <100.00%> (ø)

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