Apparent duplicates in einheitenaenderungnetzbetreiberzuordnungen are dropped

[nesnoj]

During import into changed_dso_assignment / ChangedDSOAssignment the log says

File 'EinheitenAenderungNetzbetreiberzuordnungen.xml' is parsed.
Data is cleansed.
778 entries already existed in the database.

However - if I understand the table right - the system operator may change multiple times so it's not a good idea to solely use EinheitMastrNummer as primary key:

open-MaStR/open_mastr/utils/orm.py

Lines 795 to 807 in 6bc7254

	class ChangedDSOAssignment(ParentAllTables, Base):
	__tablename__ = "changed_dso_assignment"
	EinheitMastrNummer = Column(String, primary_key=True)
	LokationMastrNummer = Column(String)
	NetzanschlusspunktMastrNummer = Column(String)
	NetzbetreiberMastrNummerNeu = Column(String)
	NetzbetreiberMastrNummerAlt = Column(String)
	ArtDerAenderung = Column(String)
	RegistrierungsdatumNetzbetreiberzuordnungsaenderung = Column(
	DateTime(timezone=True)
	)
	Netzbetreiberzuordnungsaenderungsdatum = Column(DateTime(timezone=True))

To avoid losing this data, we should think of a primary key constraint on multiple columns such as

__table_args__ = (
        PrimaryKeyConstraint('EinheitMastrNummer', 'NetzbetreiberMastrNummerAlt', NetzbetreiberMastrNummerNeu),
)

However, there could be duplicates as well so maybe it's better to include the date like this:

__table_args__ = (
        PrimaryKeyConstraint('EinheitMastrNummer', 'Netzbetreiberzuordnungsaenderungsdatum'),
)

The table was added in #510