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

Improve/fix JSON deserialization

See merge request acplt/pyaas!19

adjust indent in continuation line

adjust _construct_qualifier() and _construct_asset_administration_shell() for merged master

to fix an error showing up since mypy 0.770
aas/model/base.py:573: error: Incompatible types in assignment (expression has type "Referable", variable has type "Identifiable")

readme: Fill with content

Closes #44

See merge request acplt/pyaas!18

Add AASX reading and writing functionality

Closes #38

See merge request acplt/pyaas!16

Add py.typed file to allow typechecking against this library

See merge request acplt/pyaas!17

* Make features shorter and clearer
* Add "Getting started"
* Add "Contributing"