Feature #2521 sonarqube code smell

[hsoh-u]

Expected Differences

The changes were made based on the code small report by SonarQube. The modified code should not produce any differences at the outputs.

• Moved down "using namespace after #include statement.

• Added namespace for header files

• Removed unnecessary parentheses with return statements.

• is_leap_year.cc: separated a code block to a sub-routine to reduce the depth of if/for statements.

• Used nullptr instead of 0

• And others for SonarQube

• Do these changes introduce new tools, command line arguments, or configuration file options? [No]
  
  If yes, please describe:
  

• Do these changes modify the structure of existing or add new output data types (e.g. statistic line types or NetCDF variables)? [No]
  
  If yes, please describe:
  

Pull Request Testing

• Describe testing already performed for these changes:
  

I did an unit test manually at seneca and compared the outputs with nightly build.

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

The GHA without any differences

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes or No]

• Do these changes include sufficient testing updates? [No]

• Will this PR result in changes to the test suite? [Yes or No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Please complete this pull request review by [Fill in date].
  

Pull Request Checklist

See the METplus Workflow for details.

• Review the source issue metadata (required labels, projects, and milestone).
• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
• Define the PR metadata, as permissions allow.
  Select: Reviewer(s)
  Select: Organization level software support Project or Repository level development cycle Project
  Select: Milestone as the version that will include these changes
• After submitting the PR, select Development issue with the original issue number.
• After the PR is approved, merge your changes. If permissions do not allow this, request that the reviewer do the merge.
• Close the linked issue and delete your feature or bugfix branch from GitHub.

[JohnHalleyGotway]

I approve of these changes. I see that GHA ran and only flagged one difference from a PR unrelated to these changes.