Fix/fourrier weight

Issue/feature addressed

This MR fix the missing factor 2 in the integration weight of Fourier modes resulting in a factor sqrt(2) difference in the L2 error norms of many tests.

Proposed solution

Implementation

Tests

Suggested reviewers

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

Notes

Fixes #29 (closed).

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).