Fix: Crash when export window is closed without specifying a file, added R_in

🐛 fix(data_processing.py): fix typo in advanced data keys and add R_max to advanced data ✨ feat(MainWindow.py): add support for exporting advanced data to a text file 🎨 style(MainWindow.ui, UI_MainWindow.py): change the display of advanced data labels to use instead of 〈R〉 and σ instead of σ〈R〉 The README.md file now reflects the fact that the tool can now read .stl files. The advanced data keys in data_processing.py have been fixed to use the correct symbols and R_max has been added to the advanced data. The MainWindow.py now allows the user to export the advanced data to a text file. The display of advanced data labels in MainWindow.ui and UI_MainWindow.py has been changed to use instead of 〈