Blackholes swallowing - Gas case

This implements the following:

• Black holes carry metal information. That is obtained at brith time and written to snapshots.
• Added 1 new loop over the gas neighbours of BHs to tag the gas particles that will be swallowed.
• Added a field to the gas particles to carry the swallowing information.
• Added a set of tasks to process the particles that have been tagged for swallowing.
• Reduced the number of BH tasks by only creating them for cells that actually contain black holes.
• Added the communication and dependencies that are needed for the swallowing.
• Implemented the EAGLE model for BH swallowing of gas.
• In debugging mode, copy the (const) pointer to the task that is run into the runner. Helps seeing where we are.