Add unified structural/diagnostic PREPBUFR checker and update README

- Updated `check_prepbufr.py` to support two-level validation:
  (A) Structural GSI-compatibility checks (minimal header/coords/useful vars)
  (B) Full diagnostic mode (events, time window, units, oberrs, ADPSFC fields)
- Added automatic pressure-unit detection (hPa/Pa/cb)
- Added aggregated and detailed CSV reporting (`--csv`, `--report-csv`, `--vars-csv`)
- Added consistent exit codes: 0 for structural OK, 1 for structural failure
- Improved progress reporting and ‘--where’ sample output
- Harmonized logic previously split between `check_prepbufr_gsi_compat.py` and the full checker

- Updated README.md:
  * Documented both usage modes (GSI-compat and full diagnostic)
  * Added interpretation of "Status estrutural (GSI)" and "Status diagnóstico"
  * Documented unit ranges, pressure-unit inference, ADPSFC checks, and oberrs
  * Added CSV format descriptions and troubleshooting guidelines
  * Provided clear, reproducible examples with command lines

This commit consolidates PREPBUFR validation into a single unified tool
and provides complete documentation for operational and research workflows.

Author: joaogerd