Titration data export/load

Add a load/save function to the titration data widget, so that the data can be saved readily. Should use a pickle file for this so that users can't accidentally change data along the way

Start by locating the titration data widget and its existing data-handling entry points. Check how the widget exposes load and save actions, then verify the workflow with a pickle file; done means users can save and reload titration data without unintended changes.

Written by the indexing model from the issue text.