Respect max speed values in weight streetnet

[mpadge]

Current "motorcar" weighting defaults to weighting profile, which defines maximum speed values for each way type. Routing times would be much more accurate if local max speed values were used instead. Context: https://github.com/ATFutures/m4ra/issues/3

• [x] Include maxspeed key-value pairs in addition to current list of key-val pairs
• [ ] Implement routine to extract mean or median maxspeed values for each way type, to use as default on ways without specified values
• [x] Use resultant values to define times for each edge during ˋweight_streetnetˋ.

[mpadge]

That actually already exists as part of the "weight-streetnet-times.R" code. The problem is that the current routine constructs a vector of potential values from (profile for way type, OSM maxspeed values, profile for surface type), and then selects the lowestof those. This routine then just needs to be modified to replace the lowest value for "motorcar" profiles with OSM values where those are given.