Use std::isnan instead of isnan

[Pasarus]

Description of work

Mantid can fail to compile on slightly more up-to-date operating systems (OS), this will fix this.

Summary of work

Added std:: in front of some isnan calls in 2 files.

Purpose of work

Work was completed because I was trying to compile mantid, and I've posted this PR so others won't need to do the same.

There is no associated issue.

Further detail of work

Added std:: in front of some isnan calls in 2 files.

To test:

There are no instructions to test, as testing is compiling, and it should have compiled in the Jenkins workflows.

This does not require release notes because it is not user-facing and is just about compiling Mantid on a more up-to-date OS

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Reviewer

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[thomashampson]

I approve these changes.

[SilkeSchomann]

@Pasarus Please add the 6.13 milestone.

[Pasarus]

I cannot

[SilkeSchomann]

I cannot
Ah, ok. Done.