New bd_retrieve...() API to deprecate retrieve_data() & retrieve_adv_search()

[katrinleinweber]

This PR implements https://github.com/TIBHannover/BacDiveR/issues/96#issuecomment-455848352.

I have...

• [x] ... run make check in the terminal and saw now errors.
• [x] ... added/updated tests to https://github.com/TIBHannover/BacDiveR/tree/master/tests/testthat/ for new features or bug-fixes.
• [x] ... updated the changelog / NEWS.md in the Unreleased section, if my PR introduces user-facing changes.
• [x] ... read (at least) the license and agreed that it will apply to my contribution as well.

If this is a release PR, additionally

• [x] Move Unreleased changes in NEWS.md to dedicated section
• [x] Copy those & the new version number to new GitHub release, together with the README.md's 1st paragraph
• [x] Increment the [SemVer.org] number in the DESCRIPTION file.

[katrinleinweber]

The Travis error does not result from any code changes. Maybe from a TeX problem in the documentation output (incl. BacDive data downloads), or some other problem on Travis' side. Merging & releasing anyway, in order to then continue with #103.