Fix bug & Update the user guide with a sample script

Issue/feature addressed

1. A bug is fixed in the wallNormalData FieldConvert module.
2. The user guide is updated with a sample Python script to get the profile for visualize and postprocessing.

Proposed solution

The dimension cannot be obtained in the omdule constructor. It is now moved to v_Process()

Implementation

Simple fix.

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

Notes

Please add any other information that could be useful for reviewers.

Checklist

• [ ] Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• [ ] Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).