GUI: Added new dialogue box for assigning unit cell
Resolves #97 (closed) and #129 (closed)
- New class UnitCellDialog prompts for a name for the unit cell and offers a drop menu to select the peak collection to assign it to.
- Made the function of the "Accept solution" button clearer
- Changed functioning of Experiment::assignUnitCell to have a name argument with a default
- Changed Peaklist -> Peak colleciton in ListNameDialog for clarity