`download_*` optional parameters

[mitchellmanware]

• [x] date_start & date_end -> date = c()
• [x] year_start & year_end -> year = c()
• [ ] etc

[mitchellmanware]

https://github.com/NIEHS/amadeus/commit/3e28b35226d318f9fdcc1fdea7299d2ec327b3a3