nMFI normalisation type

[Fersoil]

• Added function get_nmfi
• modified process_plate function to handle new normalisation type
• added tests and docs for the get_nMFI function
• added tests for process_plate

This PR should be merged after a successful review of #171

[codecov]

Codecov Report

Attention: Patch coverage is 96.20253% with 3 lines in your changes missing coverage. Please review.

Project coverage is 87.10%. Comparing base (dc478e9) to head (f24bd5a).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
R/process-plate.R	92.68%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #174      +/-   ##
==========================================
+ Coverage   86.60%   87.10%   +0.50%     
==========================================
  Files          13       14       +1     
  Lines        1515     1574      +59     
==========================================
+ Hits         1312     1371      +59     
  Misses        203      203              

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[ZetrextJG]

Nice work. I like that the get_nmfi function is kinda separated. I would consider extending the vignettes and state there clearly what nMFI is and how can a user get that data.

Besides that i have two minor nitpicks into the code, not a big deal, but I would like to

[ZetrextJG]

Nice to see vignettes. I added some code concerns that I have not caught in the first round of review

[ZetrextJG]

LGTM