Adapt constructor of model.Key

[dxvidnrt]

Version 3.0 changes the constructor of model.Key.
This PR implements the changes by:

• Changing type of parameter _type to model.KeyTypes from model.KeyElements
• Removing boolean parameter
• Changing type of parameter value from str to Identifier

Also a mixup of indices in test_base was corrected.

[dxvidnrt]

@s-heppner Ready for merge

[s-heppner]

This seems to not touch model.Key, just the examples and the tests?

[dxvidnrt]

I forgot to change the type of Key/value to model.Identifier, this is fixed now.

[dxvidnrt]

@s-heppner
I am wondering if the implementation of __str__ in base.Key is correct or if it should be switched to something like

def __str__(self) -> str:
    return self.value.__str__()

same goes for Key.__repr__ due to value being of type Identifier instead of str.
Else I think the code is ready to merge

[s-heppner]

I think __repr__ is fine for the time being.

def __repr__(self) -> str:
        return "Key(type={}, value={})".format(self.type.name, self.value)

Here, the .format()-function would convert self.value to a string, if it wasn't already one.

Nevertheless, I agree, it's better to change __str__ as you described.

[dxvidnrt]

@s-heppner The last commit implements the change in Key.str()