fix: restore default CO2 characterization for emission flows

[TimoDiepers]

Problem

Since 1.4.0, create_characterization_functions_from_method() assigns no characterization function to ordinary CO2 emission flows (Carbon dioxide, fossil with categories ('air', 'non-urban air or from high stacks') etc.). characterize() then silently skips them, so dynamic LCIA (metric="GWP" or "radiative_forcing") misses ~90% of the climate impact. Downstream, every current bw_timex install is affected, since TimexLCA.dynamic_lcia() relies on this default mapping.

Root cause: the emission branch required "in air" in node["categories"], which only holds for the natural-resource flow Carbon dioxide, in air. Real emissions have ('air',), ('air', 'urban air close to ground'), ... and there was no else branch. Carbon dioxide, from soil or biomass stock was unmapped too, because the uptake check matched "soil" against categories while that flow carries "soil" in its name and ('air', ...) categories.

1.3.1 and earlier had a catch-all else and were fine.

Fix

Uptake is detected explicitly, everything else falls through to the emission function:

categories = node.get("categories") or ()
is_soil_uptake = "soil" in categories
is_uptake_resource = "in air" in categories and node.get("type") == "natural resource"

if is_soil_uptake or is_uptake_resource:
    if characterize_uptake:
        characterization_functions[node.id] = co2_uptake_func
else:
    characterization_functions[node.id] = co2_func

characterize_uptake=False now suppresses only the uptake functions, not all CO2 characterization.

Tests

New tests/test_default_characterization_functions.py builds a synthetic biosphere + method with bw2data.tests.bw2test, so no ecoinvent is needed:

• 6 parametrized CO2 emission flows (all air subcategories, non-fossil, from soil or biomass stock) map to characterize_co2
• emission flows stay characterized with characterize_uptake=False
• Carbon dioxide, in air (natural resource) and CO2 to soil map to characterize_co2_uptake, and are absent when characterize_uptake=False
• every method flow gets a function
• end-to-end: dynamic GWP100 with fixed_time_horizon=False is within 5% of the static score of the same inventory

Verified failing before the fix (KeyError on the CO2 emission flows; end-to-end 551 vs. 22245 expected) and passing after. Full suite: 94 passed.

Version bumped to 1.4.1 with a changelog entry.