remove _unwrap() as it produced error messages that weren't helpful

add _get_text_mandatory_mapped() and _get_attrib_mandatory_mapped()
These are helper functions for retrieving an attribute or the text of
an xml element and then using the value as the key in a specified dict.
These are useful since they will raise a helpful error if
the attribute or text does not exist in the dict as a key.

add _get_child_mandatory() + _get_attribute_mandatory()
Functions for retrieving a mandatory child element or a mandatory attribute
and raising an error with a helpful message if the child/attribute doesn't exist.

_get_text_mandatory()'s signature is now similar to _get_child_mandatory()
and _get_attribute_mandatory(). It won't accept None anymore, and it now
also raises a KeyError instead of a TypeError.

change _failsafe_construct()
This function will now handle Key and Value errors.
It will now print the whole error cause chain.

change constructor functions depending on these helper functions respectively

add a check if type of last key matches reference type when constructing
AAS References

adjust indent in continuation line

adjust _construct_qualifier() and _construct_asset_administration_shell() for merged master

some minor changes mentioned in the meeting yesterday

shorten read_xml_aas_file()
minor changes to helper functions

remove #type: ignore
prefix constructor functions with _

add conceptDescription + identification parsing

prefix some function parameters with _ to suppress an unused variable warning
remove unused typing.{Type,Tuple} imports
fix reference before assignment warning in construct_lang_string_set

Seems like this missing line was the only thing that made the pipeline
fail, considering that I only added docstrings. I didn't know that it
would do this.

We have obtained the explicit agreement of Leon Mauritz Möller and
Sebastian Heppner to publish their contributions under the Apache
License (Version 2.0).

model.security, submodel: create placeholder class Security and comment the rest and fix building error

First step towards #10

model: separate paramter of initializer with new lines, rename util.py to base.py, fix ModellingKind TYPE to TEMPLATE

after having implemented most classes following the UML graphs, now
completely rework of the classes in accordance to the detailed
descriptions of the Details of the AssetAdministrationShell (DAAS)
document

Add inheritances to class AccessPermissionRule
Modify class Security to fit the most detailed model in the document
Add class Certificate (that is not further specified yet and therefore
does nothing)

also, introduce temporary references for classes Property and Submodel,
that will be replaced by the actual references as soon as they exist, in
order to make the file executable without errors, at least with the
python interpreter

There are missing references, that are not existing yet, so it can't be
executed yet. Also, aas.py is not the least finished and will be
subjected to major overhauling soon.