Missing field `ext` in some structures

[alefcastelli]

The structures Area, LoadZone and Arc do not have the ext field present. Please add it and check if there are other structures needing this addition.

This will be needed for future package development.

[jd-lara]

Area and LoadZone could have an ext field. I don't think that Arcs need ext.

[daniel-thom]

In case it matters, all components implicitly have an ext through the internal field. You can simply call get_ext(component) and add to it.