Added TileExposure and ExportHandler classes

TileExposure is a class that represents a tile and all its buildings and their attributes. gde-core instantiates this class by querying the tables of the GDE Tiles database, bringing together data and parameters that are stored separately and also over which the user has decision capacity (e.g. give number of people for which time of the day, give total costs or structural costs, etc.). The different formats will export the contents of the TileExposure object. TileExposure is independent from the output format.

ExportHandler handles the main processing activities of the gde-exporter. The main() method in gdeexporter.py calls it using multiprocessing.Pool, parallelising by quadkey group-occupancy case. The parallelisation is carried out in this way because different output files can be created for these groups and thus there is no interference in the part of the model that each core handles.