Fix bug in aggregate_enduses

[tsmbland]

Description

There was a mistake in aggregate_enduses introduced in #703. As well as checking which commodities are enduses (using is_enduse), we also need to check that technologies in the subsector produce these commodities (i.e. positive fixed_outputs)

Fixes #707

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python -m sphinx -b html docs docs/build

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works