pickle a TransportNetwork object

[r-leyshon]

Is your feature request related to a problem? Please describe.

The expensive part of an r5py workflow are:

• building the network
• computing travel times

Being able to write pre-built networks to disk would be really useful. I recall OTP had a similar functionality.

Describe the solution you'd like

Store the built networks on disk for fast retrieval on subsequent travel time operations.

Describe alternatives you've considered

Writing the travel time matrices is fine, but revisiting old networks with new questions is what I presume to be a common requirement. Eg, I have the same area of interest but now I have new origins / destinations to compute.