[BUG] unit_cell mismatch in Configuration

[oerc0122]

Description of the error
MDANSE.MolecularDynamics.Configuration : Configuration contains a special check in __setitem__ for name=="unit_cell", however, on subclasses such as _PeriodicConfiguration this is instead an @property attribute. Meaning that there can be two instances of unit_cell (one in _variables one as _unit_cell) which do not align.

Neither seems to require/coerce to an MDANSE.MolecularDynamics.UnitCell.UnitCell class.

Describe the expected result
Only once instance/method of characterising the unit cell.

Describe the actual result
Multiple, one probably unused means.

Suggested fix
Delete unused (probably __setitem__, needs checking) variant and unify.

Additional details
N/A