This issue extends #140.

This allows tracking resources in costs objects. It's definition:

Resource: Tracker {
  standard_unit: <om2::Unit>
  externality_cost: 0 < x <= 1
  sources: [ Source ... ]
}

Source: Translatable {
  lat: <latitude>
  lon: <longitude>
  externality_cost: 0 < x <= 1
  merge_function: <sum|avg>
}

Let's go over the fields:

• standard_unit - The unit this resource tracker uses. It might be liters, kilograms, etc. See om2::Unit.
• externality_cost - A value between 0 and 1 that determines how much externalities this resource is responsible for (as a ratio). 0 would mean "has no known externalities" and 1 would be "critically damaging to the environment or humanity."
• sources - A list of known sources for this resources, ie "Forestville rock quarry" or "Amazon rainforest". Each of these has their own externality_cost which is combined with the tracker's main externality cost via the merge_function.