Improve SYNOP Section 5, 6 and 7 decoding

[Saeid008]

Summary

This pull request improves SYNOP decoding and encoding, with a focus on Sections 5, 6 and 7.

Changes

• Improve SYNOP Section 5 decoding and encoding.
• Add support for 24-hour maximum wind observations (3ddff and 4ddff).
• Improve handling of optional and missing wind direction values.
• Fix Code Table 0877 encoding for variable/unknown wind direction (99).
• Improve SYNOP Section 6 decoding.
• Add support for soil moisture observations in SYNOP Section 7.
• Add additional validation and handling for Section 7 groups.
• Add extensive regression and round-trip tests for the new functionality.
• Ignore local development backup files.

Code Table 0877

The wind direction encoder previously did not correctly preserve the SYNOP 99 value for variable/unknown wind direction.

This change ensures that:

99 → variable/unknown direction

is decoded and encoded consistently.

Testing

The full test suite passes successfully:

448 passed, 1 skipped, 37 warnings

The warnings are existing DecodeWarning messages related to SYNOP Group 7 handling and do not cause test failures.

Also verified SYNOP round-trip encoding/decoding for:

• Calm wind (00000)
• Normal wind directions
• 360° wind direction (13612)
• Variable/unknown wind direction (19906)

All tested cases round-trip successfully.

[Saeid008]

This PR also addresses the issues described in #19, particularly the missing SYNOP Section 5 supplementary groups and the inconsistent handling of the 910 gust group.

The relevant decoding and encoding support has been added along with regression tests.