Improve coding

[fa201]

• better comment the code (why, how)
• better split or merge functions
• use os.path.join() for file path
• use entry.state(['disabled']) instead of state='disabled' (deprecated).
• https://codereview.stackexchange.com/questions/239821/simple-gui-unit-converter-with-tkinter. Separate widget creation and widget positioning
• Do not create variable for labels except if label should be updated by a StringVar
• Initialize StringVar with the actual value (value='blabla'). Avoid using set
• String concatenation must be done with ''.join(list_of_strings)