Give each RefinementBatch its own unit cell

Resolves #127 (closed)

The refiner had a pretty serious bug where each RefinementBatch had a pointer to the same unit cell, thus after the refinement precess, the UnitCell for all batches was just the cell for the last batch. I have fixed this by passing the UnitCellHandler object (generally owned by Experiment) to Refiner, and adding a new std::unique_ptr<UnitCell> to the CellMap for each batch.

Note that this seems to have the unintended side effect of breaking the cell symmetry during refinement.