Support for legacy file format in DNS Reduction GUI

[koshchii]

Description of work

Summary of work

Fixes #36065.

Further detail of work

This PR implements support for loading legacy file format in DNS Reduction GUI. More specifically:

• The neutron polarisation channel is determined automatically using the values of "currents" provided in the B-fields section of a legacy file
• Legacy filenames can be handled properly at the script generation stage

To test:

• Download test data:

  • Present format: Elastic_Powder_New.zip
  • Legacy format: Elastic_Powder_Legacy.zip

• Open the reduction GUI [Interfaces->Direct->DNS Reduction]

  Repeat the steps below for present and legacy format files and make sure that the outcome is identical

• Browse to the directory where the test data are located using the "Paths" tab of the GUI

• Load and select the data for reduction using the "Data" tab

• Choose the data reduction options using the "Options" tab

• Reduce the data and generate a data reduction script using the "Script Generator" tab

• Visualize the results using the "Plotting" tab

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[peterfpeterson]

The changes look reasonable and only appear to affect DNS data reduction