DicomText - treat DT (DateTime) types similarly to DA (Date) types

[howff]

Proposed Changes

Treat DT (DateTime) types similarly to DA (Date) types, not as a normal string type

Types of changes

What types of changes does your code introduce? Tick all that apply.

• Bugfix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-Only Update (if none of the other choices apply)
  • In this case, ensure that the message of the head commit from the source branch is prefixed with [skip ci]

Checklist

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• Reviewed the contributing guidelines for this repository
• Ensured that the PR branch is in sync with the target branch (i.e. it is automatically merge-able)
• Updated any relevant API documentation
• Created or updated any tests if relevant
• Created a news file
  • NOTE: This must include any changes to any of the following files: default.yaml, any of the RabbitMQ server configurations, GlobalOptions.cs
• Listed myself in the CONTRIBUTORS file 🚀
• Requested a review by one of the repository maintainers

Issues

If relevant, tag any issues that are expected to be resolved with this PR. E.g.:

• Closes SR anonymiser creating invalid DICOM date values #1561

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (badda4b) 67.87% compared to head (0e21d63) 67.87%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1562   +/-   ##
=======================================
  Coverage   67.87%   67.87%           
=======================================
  Files         185      185           
  Lines        6546     6546           
=======================================
  Hits         4443     4443           
  Misses       2103     2103           

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