Use std::pair instead of QPair in utility files

Description of work

This PR removes the use of QPair for std::pair in the WorkspaceUtils and InterfaceUtils files used in much of the Spectroscopy code.

To test:

Code review and follow these tests https://developer.mantidproject.org/Testing/Inelastic/DataProcessorTests.html

Reviewer

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Code Review

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Does the code conform to the coding standards?
Are the unit tests small and test the class in isolation?
If there is GUI work does it follow the GUI standards?
If there are changes in the release notes then do they describe the changes appropriately?
Do the release notes conform to the release notes guide?

Functional Tests

Do changes function as described? Add comments below that describe the tests performed?
Do the changes handle unexpected situations, e.g. bad input?
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mantidproject / mantid