Refactor of peak merging

• Peak merging now has its own object, PeakMerger to make it more consistent with the parameter refactoring in !288 (merged)
• Propagated this change to the GUI, and went through the peak exporting process. Previously it completely regenerated the merged peak collection before saving, which may have been why the saved statistics and peak lists were inconsistent with the GUI. Exporting and saving now uses the same merged data and calculations as the core.
• Removed the "autoindexing" peak collection, which was annoyingly always at the top of the combo and never used.
• Fixed a bug that prevented filtering (and therefore autoindexing) from working

This merge should do something about #281 (closed) and #298 (closed), but I will leave those open until the testers can confirm they are functional.