Show power spectral in one area and do fields averaging

Issue/feature addressed

Two features are added in this merge request.

1, To analyze the characteristic spanwise wavelength of the flow, the energy distribution on the spanwise wavenumber space can be output. The most energetic wavenumber is the characteristic wavelength.

2, Field averaging can be done using FilterAverageFields during simulation, but cannot be done in the postprocessing stage.

Proposed solution

1, The power spectral in a given box region is output.

2, Fields averaging is implemented as a FieldConvert module.

Implementation

Tests

Suggested reviewers

Notes

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).