Pedestrian pipes

[barneydobson]

Description

• It is common(ish) for pipes to not be under roads and instead along walkways. This method downloads the walking network and composes it with the drive network.
• A user may specify allowable network_type in topology_derivation.allowable_networks. The downloader (prepare_data.download_street) retrieves and composes all of these.
• Only the drive network is assumed to contribute to impervious area and so have others lanes set to 0.

Other changes:

• If verbosity is enabled, graph geojsons are saved after every graphfcn (this has been massively helpful for debug).
• Some other various small tidying/bug fixing.

Discussion points:

• We might not want all pedestrian networks to be considered, as this will potentially put a lot of pipes and manholes in dumb locations. The only simple solution I can think of would be to add some weight parameter to these networks to avoid them if possible, but it doesn't feel very satisfying. @cheginit - wonder if you have any thoughts on this? - now in Remove nodes that are likely not part of the network #161
• 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. - now in Naming convention of street -> network #162

Fixes #128

[dalonsoa]

I've a couple of comments and suggestions. Otherwise, I see significant amount of code changed, but no new tests added or updated...