Add "lastChangedDate" to FileHeader

The FileHeader has an attribute for "date", which is described as "User specific date and time recommended: YYYY-MM-DDThh:mm:ss."

I would suggest to add an optional attribute to the FileHeader called "lastChangedDate", of type "dateTime", with description "Date and time of the last change." At the same time, I'd suggest to change the description of the "date" to "User specific date and time recommended: YYYY-MM-DDThh:mm:ss. It is recommended to use this attribute for the date and time of scenario creation."