Fix typo in user-guide and fix bug for parallel-in-time FieldConvert

Issue/feature addressed

Fix typo in documentation and fix bug for parallel-in-time FieldConvert when no subdirectories are specified. The bug fix allows to convert in parallel multiple solution checkpoints without specifying subdirectories

mpirun -np <nproc> FieldConvert --npt <nproc> -f test.xml test_1.chk test_1.dat

This will process file test_1.chk to test_1+<nproc>.chk in parallel.

This MR is partially split from !1613 (merged)

Proposed solution

Implementation

Tests

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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).