Refactored unit conversion doesn't work within species

[danielhuppmann]

Following the refactoring of the convert_unit() function for GWP (see #361), converting the physical unit of a species (without changing the species) does not work any more unless also providing a GWP-context.

TEST_DF = pd.DataFrame([
    ['model_a', 'scen_a', 'World', 'Primary Energy', 'Mt CO2/yr', 1, 6.],
],
    columns=pyam.IAMC_IDX + [2005, 2010],
)
pyam.IamDataFrame(TEST_DF).convert_unit('Mt CO2/yr', 'Gt CO2/yr')

raises an UndefinedUnitError.

This seems like a bug, because a context is shouldn't be necessary from first principles.

[khaeru]

Good catch. It looks like this wasn't covered by any existing test, so the first step will be to add one. Will open a PR.

[danielhuppmann]

closing as implemented