Should the abroad methods be retired?

[kndehaan]

During the effort in this PR of removing outdated recursive methods, the question was raised whether we should remove 'abroad' methods. This was an attribute assigned to nodes, but it's currently not used anymore. Therefore, the related methods could be removed.

The attribute and calculation methods might however still be useful for determining the full bio footprint of a region, as it could be used to assign a certain loss penalty when primary bio carriers are imported, instead of produced domestically.

This should be further investigated when we'll revise the recursive methods.

[github-actions]

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