Abgleich

- FEATURE: `zfs-auto-snapshot` can be told to ignore backup datasets on the target side, see 3.
- FEATURE: `samba` can optionally be told to NOT share/expose backup datasets on the target side, see 4.
- FEATURE: `ssh`-port on source and target becomes configurable, see 22.
- FEATURE: New configuration fields for `source` and `target` each: `processing`. They can carry shell commands for pre- and post-processing of data before and after it is transferred via ssh. This enables the use of e.g. `lzma` or `bzip2` as a custom transfer compression beyond the compression capabilities of `ssh` itself. See 23.
- FEATURE: `abgleich clean` can also remove snapshots on `target` but only if they are not part of the current overlap with `source`. The behavior can be controlled via the new `keep_backlog` configuration option, see 24 and 25.
- FEATURE: Configuration module contains default values for parameters, making it much easier to write lightweight configuration files, see 28. The configuration parser now also provides much more useful output.
- FEATURE: `abgleich tree` and `abgleich compare` highlight ignored datasets.
- FEATURE: Significantly more flexible shell command wrapper and, as a result, cleaned up transaction handling.
- FEATURE: Python 3.9 and 3.10 compatibility.
- FIX: Many cleanups in code base, enabling future developments.

- FIX: `tree` now properly checks if source or target is up, depending on what a user wants to see, see 20.
- FIX: All `abgleich` commands can properly initialize (instead of crashing) if the target tree is empty, see 19.
- FIX: `tree` shows message if there is no tree instead of crashing, see 18.

- FIX: Development installs with `make install` do no longer fail, see 17.
- FIX: Python up to 3.7.1 does not handle type checks for `OrderedDict` properly and either causes exceptions or crashes. Fixed with workaround, see 16.

- FEATURE: Version shown in GUI
- FEATURE: Version exposed through `--version` option on command line
- FEATURE: While `{zpool}{/{prefix}}` is included in all operations by default, this can be deactivated by setting the new `include_root` configuration option to `no`, see 14.
- FIX: If a remote host is not up, provide a proper error and fail gracefully, see 15.

- FEATURE: Improved labels in wizard GUI
- FEATURE: Significantly improved German translation
- FIX: Importing `CLoader` and `CDumper` from `pyyaml` caused crashes if they were not present in `pyyaml` packages, see 2.
- FIX: The `mountpoint` property of ZFS datasets is no longer assumed to be present at all (set or unset). This allows to handle ZVOLs without crashing, see issue 6.
- FIX: Versions 0.0.2 and 0.0.3 were completely ignoring ZVOLs when looking for snapshot tasks, see issue 10.

- FEATURE: Added wizard GUI for backup tasks (`snap`, `backup`, `cleanup`)
- FEATURE: Added new configuration options (`always_changed`, `written_threshold`, `check_diff`) for detecting snapshot tasks
- FEATURE: CLI and GUI translations (i18n)
- FIX: Added missing type checks