Datetime for date class

[kristinwo]

The output of the date in the calendars in the GUI is a datetime object. We should maybe make it possible to give either a datetimeobject or year, month etc. to the date class, so that we can still use the approach with terminal, but that it is also easier in the GUI for us to save the date?

[ddd42-star]

I think we can use the method strftime(), change it in a string that we can split and access the year, month and day for our object date. What do you think?

[ddd42-star]

At the end was a string that we traformed in an object date from the str.split('-')