Use `external_inds` and `edge_inds`/`internal_inds` as alternatives to `siteinds` and `linkinds`

[Matt Fishman]

Use external_inds and edge_inds/internal_inds instead of siteinds and linkinds, or as alternative names through aliases.

[Joseph Tindall]

Maybe we could use external_indsnetwork and external_inds to differentiate between outputting an index network vs outputting a collection of indices.