Add get_covidcast_data() function

[LucieContamin]

• add get_covidcast_data() and fix_negative_counts() with dependencies and documentation
• comment validation_plot R script to avoid problem of installation of the package
• To add the new functions:
  • add documentation (please @shauntruelove , can you review the update and please feel free to edit/fix)
  • rewrite the "missing date" code part in the fix_negative_counts() function to remove rlang dependencies that was causing conflict with data.table as both have functions with the same name (:=). (please @shauntruelove , can you review the update and please feel free to edit/fix)
  • add dependencies in DESCRIPTION, NAMESPACE
  • add some global variables

[LucieContamin]

Still need to update the documentation of the README and Vignettes to add the get_covidcast_data() function information [DONE]