Reintroduce Command Group Tags (Group 0000)

[mlibanori]

This PR restores the command group tags (group 0000) that were removed in PR #316.

While it's true that the current project does not directly handle DIMSE messages or use these tags, it's important to highlight that this library may serve as a foundational component in other projects — including those implementing the DICOM Upper Layer Protocol, which facilitates network communication in the DICOM standard.

The command group tags are defined in the DICOM standard for a reason. Although they do not appear in standard DICOM files, they are essential for parsing and constructing DIMSE messages, which operate over the DICOM Upper Layer Protocol (DUL). Removing them entirely reduces the flexibility and applicability of this package in broader DICOM-related use cases.

Changes included:

Re-added the group 0000 (command group) tags to the tag definitions.

Added tests to validate the presence and structure of command group tags.