fix: stdValue in TMTV mode

[Celian-abd]

Context

We find out that the stdValue return in the cachedStats for the segment in TMTV mode is wrong. We compared it with the same image and ROI and found two different result.

What we have:

What we should have :

Changes & Results

• Fix stdValue calulation
• Optimize by calculating the mean only once

Testing

Checklist

PR

• [] My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• [] My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• [] The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• [] OS:
• [] Node version:
• [] Browser:

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