generalise more complex configuration of features

[fredshone]

We currently support some simple "object" features, which generally look for some tag or feature of the object (such as levels or the polygon area):

• units
• levels
• area
• floor area

and more complex features, which require some calculation between objects (ie distance):

• transit_distance

In all cases these are currently configures by the features_config:
"features_config": ["units", "levels", "area", "floor_area", "transit_distance"]

We would like to generalise the transit_distance feature to (for example) calculate:

• nearest shop
• nearest hospital
• etc

In which case we would need to give it it's own config group, eg:
"nearest_config": ["transit_stop", "leisure", "health", "education"]

This pattern would then be used for more complex features, such as:

• nearest_3_config
• nearest_neighbour_config
• 1km_count
• etc

Calculating more complex features is likely to benefit a lot from a good spatial library.

Also note that the assumption is that all objects used for complex features will be included in the activity_config, eg:

"railway": {
  "station": ["transit_stop"],
  "stop": ["transit_stop"],
  "tram_stop": ["transit_stop"]
},
  "highway": {
  "bus_stop": ["transit_stop"]
},
  "public_transport": {
  "station": ["transit_stop"]
}

Transit stops are currently held in memory by a list: build.ObjectHandler.transit_stops and also in memory in a spatial index using build.ObjectHandler.points becasue they are included in the activity_config.

1. this duplication should be removed
2. ideally the complex features will get a more generalised config gtroup pattern and feature extraction pattern