Last-minute fixes, robustness improvements, and documentation

[amotl]

• URL vs. path computation fixes
• Data directory export: Verify parameters bucket-id and measurement They must be given, and not empty. Otherwise, no data slot can be addressed for input.
• Documentation: InfluxDB API first, data directory second
• Add Codecov configuration, with relaxed settings
• Be --verbose by default
• Chore: Don't log stderr from subcommands when empty

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 42.85714% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 79.46%. Comparing base (11391de) to head (4bdefa6).

Files	Patch %	Lines
influxio/core.py	14.28%	6 Missing :warning:
influxio/adapter.py	0.00%	1 Missing :warning:
influxio/util/common.py	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #107 +/- ## ========================================== - Coverage 79.87% 79.46% -0.41% ========================================== Files 12 12 Lines 626 633 +7 ========================================== + Hits 500 503 +3 - Misses 126 130 +4 ``` | [Flag](https://app.codecov.io/gh/daq-tools/influxio/pull/107/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/daq-tools/influxio/pull/107/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools) | `79.46% <42.85%> (-0.41%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.