New CRAN Release - Bug fixes, colour options and case study

Feature updates

• Added viridis_palette option to all plotting functions to allow the colour scheme to be set by the user.
• Added ability to handle categories in the legend for map_tb_burden
• Added new case study exploring case fatality rates.

Package updates

• Added improved tests in response to WHO data updates. See here
• Added an additional method for downloading data in reponse to test failures
  on CRAN. See here
• Added skip on CRAN for data backend related tests.
• Added Makefile at top level and for data-raw to build the package.