Add setting dialog with setting for global number display accuracy

[moritzwade]

Currently, there is no "right" place to put such a setting.

Probably best would be to add a dialog somewhere like the "File" menu -> "Settings" (for program-wide settings that don't have to do with an individual model, etc.)

Such info would then be saved in the QSettings object that is already used to save the list of recent files (all of this is in BioPARKIN.py).