Fix for DNS Reduction interface not opening

[SilkeSchomann]

Description of work

Summary of work

The DNS Reduction interface did not open as the import for the interface failed. I have added a __init__.py file to the directory qt/python/mantidqtinterfaces/mantidqtinterfaces/dns_single_crystal_elastic so that it is a python package.

Fixes #38249

To test:

Navigate to the Direct menu and click on DNS Reduction.

This does not require release notes because a regression for an existing interface.

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[MohamedAlmaki]

I've tested it on my Windows machine and the interface opens as expected