Transltes interp_coh to data.table

[cimentadaj]

• Translates tidyverse code to data.table
• Makes sure all syntatic sugar of data.table is cleaned and doesn't mess up devtools::check
• Adds simple tests for midyear = TRUE/FALSE such that it doesn't fail

Things to do:

• Document some arguments from the functions that weren't included in roxygen. Here's the R CMD Check:

  Undocumented arguments in documentation object 'census_cohort_adjust'
   ‘pop’
  
  Undocumented arguments in documentation object 'interp_coh'
   ‘lxMat’ ‘age_lx’ ‘dates_lx’ ‘...’

I didn't add this because I don't know if anyone is working on this but we should add . <- NULL inside interp_coh_download_mortality to avoid this in R CMD check:

interp_coh_download_mortality: no visible binding for global variable
     ‘.’
   Undefined global functions or variables:
     .

[timriffe]

Thanks for the observations, getting a graceful pass locally