Added space group selection to UnitCellDialog

Resolves #114 (closed)

When assigning the unit cell to a peak collection via the "assign" button in the indexing tab of the GUI, you can now also select the space group from a drop menu. Note that the contents of this drop menu are strongly contingent on the quality of the indexing.

Note that the UnitCell::compatibleSpaceGroups algorithm does in fact detect the centering, as evidenced by the 'Bravais type' column in the GUI, but more often than not, the unit cell with the correct dimensions will not have the correct centering, and the search requires some tweaking of parameters.