Ds18b20-datalogger

- Grafana Dashboard: Trim identifying information: `id`, `uid`,
`version`. Thanks, bee-mois.
- Emit user admonition on `make-config` subcommand. Thanks,
bee-mois.
- Data Model: Make MQTT settings optional, to allow running the
data logger without configuring them.
- Allow defining per-sensor offsets within the configuration
file. Thanks, bee-mois.

- Tests: Make sensor tests work, using a fake sysfs filesystem
- Tests: Added basic test case for CLI interface
- Removed support for Python 3.7
- Defined MQTT and sensor configuration separately from implementation.
The data logger uses a YAML file now, for example like `etc/mois.yaml`.
- Added subcommand `make-config`, for creating a configuration blueprint
- Added subcommand `make-dashboard`, for creating a Grafana Dashboard
- Added subcommand `read`, for acquiring and displaying a reading on STDOUT

- Publish as `ds18b20-datalogger` package

- Make it work, and add documentation.