Extended Matpower Format - merging component data

[rb004f]

The extended matpower format only supports adding new data to standard matpower components (buses, generators, and branches).

[ccoffrin]

@rb004f, shall we close this issue or is this still a feature you would like to see?

[rb004f]

I suggest leaving it open as a minor issue. For most components, we don't need a merge - just define the components in a single matlab table. However, for the "special" PowerModels.jl component types (loads, shunts, and ne_branches), we can not currently augment the data through the extended matpower format.

[ccoffrin]

Closing in favor of #840 which explains the challenge in a bit more detail.