Add backend methods open() and close()

[frankenjoe]

Closes #199

Adds audbackend.Base.open() and audbackend.Base.close(), and reintroduces the parts of developer guide that were commented in #200.

In the usage section and developer guide we were calling backend.exists("/") on non-existing repositories to prove if the repository was deleted. I removed those statements since it is not possible to open a connection to a repository that does not exist.

DOC

Usage

Developer guide

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.0%. Comparing base (332845c) to head (6e1c06a).

Additional details and impacted files

| [Files](https://app.codecov.io/gh/audeering/audbackend/pull/204?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering) | Coverage Δ | | |---|---|---| | [audbackend/core/api.py](https://app.codecov.io/gh/audeering/audbackend/pull/204?src=pr&el=tree&filepath=audbackend%2Fcore%2Fapi.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering#diff-YXVkYmFja2VuZC9jb3JlL2FwaS5weQ==) | `100.0% <100.0%> (ø)` | | | [audbackend/core/backend/artifactory.py](https://app.codecov.io/gh/audeering/audbackend/pull/204?src=pr&el=tree&filepath=audbackend%2Fcore%2Fbackend%2Fartifactory.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering#diff-YXVkYmFja2VuZC9jb3JlL2JhY2tlbmQvYXJ0aWZhY3RvcnkucHk=) | `100.0% <100.0%> (ø)` | | | [audbackend/core/backend/base.py](https://app.codecov.io/gh/audeering/audbackend/pull/204?src=pr&el=tree&filepath=audbackend%2Fcore%2Fbackend%2Fbase.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering#diff-YXVkYmFja2VuZC9jb3JlL2JhY2tlbmQvYmFzZS5weQ==) | `100.0% <100.0%> (ø)` | | | [audbackend/core/backend/filesystem.py](https://app.codecov.io/gh/audeering/audbackend/pull/204?src=pr&el=tree&filepath=audbackend%2Fcore%2Fbackend%2Ffilesystem.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering#diff-YXVkYmFja2VuZC9jb3JlL2JhY2tlbmQvZmlsZXN5c3RlbS5weQ==) | `100.0% <100.0%> (ø)` | | | [audbackend/core/conftest.py](https://app.codecov.io/gh/audeering/audbackend/pull/204?src=pr&el=tree&filepath=audbackend%2Fcore%2Fconftest.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering#diff-YXVkYmFja2VuZC9jb3JlL2NvbmZ0ZXN0LnB5) | `100.0% <100.0%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/audeering/audbackend/pull/204/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering)

[frankenjoe]

Would be nice to discuss there already, that we also need to add the _open() and _close() methods for this backend.

Good catch, updated to;