Add support for multigraphs in PGFrames

[eugeniashurko]

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