Add support for multigraphs in PGFrames

Currently, only one edge between a pair of nodes in allowed. For some use cases, it would be useful to support **multiedges**. For example:

Alice and Bob have two relationships, `friends` and `colleagues`, with different properties. It could be represented as:

```
@source_id, @target_id, @edge_id, @type, @since
"Alice", "Bob", 1, "friends", 1999
"Alice", "Bob", 2, "colleagues", 1998
```

Where the index of the dataframe with edges is given by `@source_id, @target_id, @edge_id`