[BUG] `unit_cell` mismatch in `Configuration`

**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

Start by reading Configuration.__setitem__ and the _PeriodicConfiguration.unit_cell property, then inspect how UnitCell.UnitCell is used. Determine which representation is authoritative and whether either path performs type coercion. Done means one consistent unit_cell representation remains and the duplicate state cannot diverge.

Written by the indexing model from the issue text.