STYLE: Method local variables should not have m_ prefix

[Leengit]

Method local variables should not have m_ prefix. That prefix is reserved for class member variables.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

[N-Dekker]

Thank you Lee. Looks fine to me!

For the record, your commit could be considered a follow-up to PR #1628 commit 7f2c44c "STYLE: Use range-based loops from C++11"

[N-Dekker]

For the record,

ITK's Coding Style Guide does indeed specify, at https://github.com/InsightSoftwareConsortium/ITKSoftwareGuide/blob/ac807207e282d075714a65060b09dd15179d5dda/SoftwareGuide/Latex/Appendices/CodingStyleGuide.tex#L756

Class data members are prepended with m_ as in m_Size. This clearly indicates the origin of data members, and differentiates them from all other variables.

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