Remap node and edge properties

[deepakunni3]

@cmungall

This PR adds the ability to remap the value of a node property with the value of another existing node property, on-the-fly when loading nodes from a connected local or remote Neo4j.

The most common use case is to remap the id property of a node with another defined node property.

Similarly for edge properties.

Usage:

    # initialize NeoTransformer
    n = NeoTransformer(None, uri, username, password)

    # add remap conditions
    n.remap_node_properties.append(("id", "alternate_id"))
    n.remap_edge_properties.append(("source", "is_defined_by"))

    # load
    n.load()

[deepakunni3]

Closing in favor of #70