* Use SPDX identifier in header files
* Use SPDX identifier in rest of source files
* Fix usage of SPDX for files with 2 licenses
* Add global license file
* Fix compilation
* fix_1416_iptc_DateCreated
* Fix unit tests
* DateValue:read 2nd iteration on pre-condition
* test with ISO_8601 date format
* Use std::regex for ISO 8601 basic & extended date formats
* Use std::regex for ISO 8601 basic & extended time formats
* Add more tests & notes for DateValue & TimeValue
* Comment tests using local calendar times
* DateValue::write also adds padding to year field
Co-authored-by: Luis Díaz Más <piponazo@gmail.com>