Create LegacyNexusClasses

[rosswhitfield]

Description of work

This includes the changes from #38824 and should be reviewed after that is merged.

This creates a copy of NexusClasses called LegacyNexusClasses that uses LegacyNexus instead of NexusCpp.

This PR also switched LoadMuonNexus1, LoadMuonNexus2, LoadMuonNexus3 and MuonNexusReader from NexusClasses to LegacyNexusClasses/NexusCpp to LegacyNexus.

• windows system tests

Summary of work

Refs #38332

Further detail of work

To test:

This is a refactor with no change in functionality. The existing tests should be enough and no release notes are required.

---

Reviewer

Please comment on the points listed below (full description).
Your comments will be used as part of the gatekeeper process, so please comment clearly on what you have checked during your review. If changes are made to the PR during the review process then your final comment will be the most important for gatekeepers. In this comment you should make it clear why any earlier review is still valid, or confirm that all requested changes have been addressed.

Code Review

• Is the code of an acceptable quality?
• 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?
• Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @mantidproject/gatekeepers will take care of it.

Gatekeeper

If you need to request changes to a PR then please add a comment and set the review status to "Request changes". This will stop the PR from showing up in the list for other gatekeepers.

[sf1919]

Again I'd like for us to get #38845 in first before this one.

[github-actions]

👋 Hi, @rosswhitfield,

Conflicts have been detected against the base branch. Please rebase your branch against the base branch.

[rboston628]

Changes should move the muons code to use the LegacyNexus package. The only changes in the muon loaders is to switch dependencies and namespaces.