unittests add:
- AASReference.resolve()
- Namespace.remove_referable()
- NamespaceSet.__contains__()

changed testing of ref5. Due to #37 preventing the creation of a `Reference` without a key

Previously, how to contribute to Eclipse-BaSyx was not
very well defined.

We add an extensive contribution guideline outlining:
  - Issue creation
  - PR Workflow
  - Commit and PR Message Conventions
and reference it in the main `README.md`.
Furthermore, we move the "Codestyle and Testing" paragraph from the
main `README.md` to the `CONTRIBUTING.md`.

When deserializing XML we would previously only have a look at the first
child of an unnecessary wrapper element and ignore additional elements.
However, since some implemenetations like the AASX Package Explorer
generate schema-incompatible XML, this behavior is changed in a way such
that additional elements are no longer ignored, but also parsed instead.

Fix #71

Currently, we can pass a `failsafe` parameter in deserializing methods 
of JSON- and XML-adapters. 
The parameter determines whether a document should be parsed 
in a failsafe way. However, I cannot pass the parameter 
in the deserializing method of the AASX Reader 
because it does not have `**kwargs`. 

This commit adds `**kwargs` to deserializing methods of `AASXReader`, 
such that it is possible to pass the parameter and documents will 
be deserializend in a not failsafe mode.

If an assertion is contained within the same context as a statement,
that raises an error, the assertion is never executed, because the
raised error leaves the context.

Since a recent update, pycodestyle requires whitespaces between the last
comma and the backslash at the end of a line, where it is broken.

During construction of a `Decimal`, an `InvalidOperation` exception
may occur. Since the user only expects a `ValueError` from this function,
because all other parsers only raise `ValueError`s, the `InvalidOperation`
error-type is handled and re-raised as a `ValueError`.

Fix #99

Mypy appears to not like abstract classes in a context where only
concrete classes should be given:

```
Only concrete class can be given where
"tuple[type[Referable], KeyTypes]" is expected
```

However, the spec demands the four abstract classes
- `Event`
- `DataElement`
- `SubmodelElement`
- `SubmodelElementCollection`

to appear inside the `model.KeyTypes` Enum.
Therefore, we ignore these errors via `# type: ignore`.

The AAS Manager uses typehints to generate a list of possible types for
a value. Since `DateTimeStamp` was missing, it was impossible to store a
`DateTimeStamp` value via AAS Manager.

…comments

…more explicit