Feature/update fieldconvert utilities

Issue/feature addressed

• Update documentation for parallel-in-time processing.
• The parallel-in-time processing code base in FieldConvert has been modified to allow arbitrary directory names
• The parallel-in-time processing code base in FieldConvert has been modified to allow (colon separated) commands option.
• Add comments to the parallel-in-time processing code base in FieldConvert
• Correct a few typos in Nektar++.

Proposed solution

Implementation

Tests

Suggested reviewers

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

Notes

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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).