Support time zone information in the createdate attribute in the index

[RKrahl]

When the index is created by photoidx create, the createdate attribute of the images is initialized from get_date_time() from the photo's exif data. But this yields a naive datetime object, without any time zone information. Should also try to get the time zone information from the exif data and set it in createdate.

However, I'm not sure if there is any standard exif field where this is stored. This might result in a vendor specific solution that only works for some camera models.

[RKrahl]

Note that photo-tools relies on PyYAML for reading and writing index files. PyYAML does not handle time zone aware datetime values correctly, see this Issue in PyYAML.

[RKrahl]

An attempt to implement this has been started in the timezone branch. But it turns out that there are a couple of problems:

1. The location of time zone information in the exif data is vendor specific. I didn't found any comprehensive access method in GExiv2, though I might have overlooked something as GExiv2's documentation is sparse. So we would need to add every special case separately and still the feature would only work for a couple of particular models.
2. We need an implementation of the datetime.tzinfo class to represent timezone aware datetime values. The standard library has datetime.timezone, but this has been added in Python 3.2 only. So either we would need add our own implementation or the feature would not work with Python 2.
3. As mentioned in the last comment, PyYAML is not able to read time zone aware datetime values.

As a summary of all this, the problems seem to outweigh the benefits of this feature. I close as wontfix for the time being.

[RKrahl]

Reopen:

• the need for datetime.tzinfo is not an issue any more, since we are requiring at least Python version 3.6 in the meanwhile,
• PyYAML properly supports aware datetime values since version 5.3.

The remaining issue to be solved is to retrieve the time zone information from the exif data. But this could also be solved providing the information in command line argiuments.

[RKrahl]

Options to determine which time zone to set might include:

• Getting them out of the exif data from the image. That would be the best option, but the info is hidden in vendor specific proprietary attributes, so this is not easy to implement and would only work for a few specific camera types.
• Use the time offset between exif create date and GPS time stamp, assuming the latter to be UTC.
• Use the current local time zone as set in the environment and assume the exif create date to be the local time in that zone.
• Add a comand line argument to override the time zone.