Simplify prepare_Renviron() with usethis::edit_r_environ()

[katrinleinweber]

See prepare_Renviron() vs. usethis::edit_r_environ(). I think this would require:

1. a check for existing BacDive_… credentials
2. if not: 1st insertion of those two template lines & 2nd usethis::edit_r_environ()

• check that the latter works fine when no .Renviron file is present yet
• add dependency
• review ReadMe