ENH: Calculate network distance and export to igraph

[HTenkanen]

Relates to #52.

Adds functionality to:

• calculate the length of network edges. Information is updated to column length (reported in meters with 3 decimals).
• (optionally) return the nodes when parsing networks (notice: handling one-way/two-way streets are handled only when exporting the graph)
• export the network into igraph.Graph (support for networkx/pandana are added next)

Example:

from pyrosm import OSM, get_data
from pyrosm.graphs import to_igraph
osm = OSM(get_data("helsinki"))
# Parse nodes and edges using `nodes=True`
nodes, edges = osm.get_network(nodes=True)
# Export to igraph
g = to_igraph(nodes, edges)