co2e 0.1.0

• Initial release.
• gwp() looks up Global Warming Potential values by gas, IPCC Assessment Report (AR4-AR6), and time horizon (20, 100, 500 years).
• as_co2e() converts a mass of gas to CO2-equivalent.
• gwp_compare() compares a gas’s GWP across assessment reports.
• standardize_gas() maps non-canonical gas names to canonical gas_id values using documented naming variants (no fuzzy matching).
• Bundled datasets: gwp_values, gases, gas_synonyms, with source-by-source provenance in vignette("provenance").