Naming convention of street -> network

[barneydobson]

The naming conventions are now a bit confusing - download_street can download any network_type (including walk and cycle), and the edge_type of any downloaded network that isn't a river is given the tag street (i.e., indicating it can have a pipe under it. I propose to replace the keyword street with network, so it would be come download_network and edge_type=network.

Originally posted in #143 by @barneydobson

Any link that can carry a pipe should be referred to as network - thus most instances of the software that refer to a street (the keyword previously used) should be changed to network - unless it is explicitly referring to a street (e.g., presumably in the downloader and streetcover calculation)