Internal helper function to resolve either a node name or a
node index to a 0-based index.
.resolve_idx0_get uses get on the fastmap and expects a single value,
while .resolve_idx0_mget uses mget and can return multiple values.
.resolve_idx0_get(nm_idx_map, node_name = NULL, node_index = NULL).resolve_idx0_mget(nm_idx_map, node_name = NULL, node_index = NULL)
A fastmap mapping node names to 0-based indices from
a caugi.
Optional character vector of node names.
Optional numeric vector of 1-based node indices.