Zappend

* Made writing custom slice sources easier and more flexible: (82)

- Slice items can now be a `contextlib.AbstractContextManager`
so custom slice functions can now be used with
[contextlib.contextmanager](https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager).

- Introduced `SliceSource.close()` so
[contextlib.closing()](https://docs.python.org/3/library/contextlib.html#contextlib.closing)
is applicable. Deprecated `SliceSource.dispose()`.

- Introduced new optional configuration setting `slice_source_kwargs` that
contains keyword-arguments passed to a configured `slice_source` together
with each slice item.

- Introduced optional configuration setting `extra` that holds additional
configuration not validated by default. Intended use is by a `slice_source`
that expects an argument named `ctx` and therefore can access the
configuration.

* Improved readability of the configuration reference by using setting
categories and applied logical ordering of settings within categories. (85)

Enhancements

* Added configuration setting `force_new`, which forces creation of a new
target dataset. An existing target dataset (and its lock) will be
permanently deleted before appending of slice datasets begins. (72)

* Chunk sizes can now be `null` for a given dimension. In this case the actual
chunk size used is the size of the array's shape in that dimension. (77)

API Changes

* Simplified writing of custom slice sources for users. The configuration setting
`slice_source` can now be a `SliceSource` class or any function that returns a
_slice item_: a local file path or URI, an `xarray.Dataset`,
a `SliceSource` object.
Dropped concept of _slice factories_ entirely, including functions
`to_slice_factory()` and `to_slice_factories()`. (78)

* Extracted `Config` class out of `Context` and made available via new
`Context.config: Config` property. The change concerns any usages of the
`ctx: Context` argument passed to user slice factories. (74)

* Fixed rollback for situations where writing to Zarr fails shortly after the
Zarr directory has been created. (69)

In this case the error message was
TypeError: Transaction._delete_dir() missing 1 required positional argument: 'target_path'.

Enhancements

* The configuration setting `attrs` can now be used to define dynamically
computed dataset attributes using the syntax `{{ expression }}`. (60)

Example:
yaml
permit_eval: true
attrs:
title: HROC Ocean Colour Monthly Composite
time_coverage_start: {{ lower_bound(ds.time) }}
time_coverage_end: {{ upper_bound(ds.time) }}


* Introduced new configuration setting `attrs_update_mode` that controls
how dataset attributes are updated. (59)

* Simplified logging to console. You can now set configuration setting
`logging` to a log level which will implicitly enable console logging with
given log level. (64)

* Added a section in the notebook `examples/zappend-demo.ipynb`
that demonstrates transaction rollbacks.

* Added CLI option `--traceback`. (57)

* Added a section in the notebook `examples/zappend-demo.ipynb`
that demonstrates transaction rollbacks.

Fixes

* Fixed issue where a NetCDF package was missing to run the
demo Notebook `examples/zappend-demo.ipynb` in
[Binder](https://mybinder.readthedocs.io/). (#47)

Fixes

* Global metadata attributes of target dataset is no longer empty. (56)

* If the target _parent_ directory did not exist, an exception was raised
reporting that the lock file to be written does not exist. Changed this to
report that the target parent directory does not exist. (55)

Enhancements

* Added missing documentation of the `append_step` setting in the
[configuration reference](https://bcdev.github.io/zappend/config/).

Enhancements

* A new configuration setting `append_step` can be used to validate
the step sizes between the labels of a coordinate variable associated with
the append dimension. Its value can be a number for numerical labels
or a time delta value of the form `8h` (8 hours) or `2D` (two days) for
date/time labels. The value can also be negative. (21)

* The configuration setting `append_step` can take the special values
`"+"` and `"-"` which are used to verify that the labels are monotonically
increasing and decreasing, respectively. (20)

* It is now possible to reference environment variables
in configuration files using the syntax `${ENV_VAR}`. (36)

* Added a demo Notebook `examples/zappend-demo.ipynb` and linked
it by a binder badge in README.md. (47)

Fixes

* When `slice_source` was given as class or function and passed
to the `zappend()` function either as configuration entry or as keyword
argument, a `ValidationError` was accidentally raised. (49)

* Fixed an issue where an absolute lock file path was computed if the target
Zarr path was relative in the local filesystem, and had no parent directory.
(45)