[page 4] As users I'd would like to define mortality for 3 periods

[statnmap]

Validation

• [ ] The model accepts three period to run simulations
• [ ] The vignette documentation shows that it wrks for 3 mortalities
• [ ] The Shiny App works with 3 periods of mortality

Technicals

• Change everywhere, including in translations :
  • yearsimubegin by firstsimuperiod
  • yearsimuend by secondsimuperiod
• Add a thirdsimuperiod and connect it to the simulation model
  • Explore in "data-raw/be-page4-future.Rmd" the effect of adding a new column before including in the app
• See in traductions "Text to separate sliders in two periods – First period","yearsimubegin","2001-2050","2001-2050","2001-2050","2001-2050" "Text to separate sliders in two periods – Last period","yearsimuend","2051-2100","2051-2100","2051-2100","2051-2100"