Add support for multiple countries

[simon-smart88]

Modules/functions needing refactor:

• resp_download
• cov_bioclim
• agg_worldpop

[simon-smart88]

• Update selectinput to allow multiple countries
• Update Rmd to accept the vector
• Functions loop through countries and merge results at the end if length(countries) > 1
• What happens downstream if countries are not neighbouring?

[simon-smart88]

Resolved in b4aa4c0