How should age in weeks be handled for deleted files in delta mode?

# Question

Currently in delta mode the age in weeks metric of deleted files is handled in a confusing way. The following screenshot shows a file that was 266 weeks old in the reference map (left selector) and was deleted in the comparison map (right selector)
This is confusing as it suggests the file is currently 266 weeks old and the age has changed by -266 weeks. This can be understood as it is now 266 weeks 'younger' than before meaning before the file was 500+ weeks old, which is false

When swapping the reference and comparison maps, the files metrics change as follows:
Suggesting the file is now 266 weeks old and was 0 weeks old (which makes more sense as one map does not include the file)

**Is this behaviour wanted or how should deleted files be handled?**
My suggestion would be that if the comparison map does not include the file, the metric simply includes a '-' to indicate it is not there, while keeping the delta value. This would make it more clear that it has been deleted and now does not have an age.

## Preparation

Please check these resources first before posting a question

* [x] I searched for answers in [older issues](https://github.com/MaibornWolff/codecharta/issues?utf8=%E2%9C%93&q=)
* [x] I looked into the [README.md](https://github.com/MaibornWolff/codecharta)
* [x] I read the [website](https://maibornwolff.github.io/codecharta/)

Start by tracing how delta mode represents the age-in-weeks metric when a file exists only in the reference or comparison map. Review the README.md and website context, then compare the two examples in the issue. Done means the project has a decided and consistently documented representation for deleted files, with the delta remaining understandable.

Written by the indexing model from the issue text.