Copy the unused input properties into the output trace

[leo-desbureaux-tellae]

Problem exposition

Today, the unused fields of the inputs properties are lost: they are not present in the output trace.

For instance, a synthetic population may contain socioeconomic profiling of the users. It could be nice to keep these in the trace geojson.

Proposed solution

Add a new attribute in the Agent class that stores the unused properties, and use it when generating the trace.