Improve harvesting routine

[SimonPiCarter]

• [x] Add slots to resources (number is defined from radius)
• [x] Each slot is taken upon arrival by an harvester
• [x] Add step to take and restore slot on resource
• [x] make harvest routine take and restore slots (careful about ALWAYS restoring slots : use clean up), make sure clean up work correctly when harvester dies
• [x] scan for slot when chosing target resource AND just before taking the slot, target becomes the slot position
• [x] need to have state in step to avoid double lock of the same slot
• [x] if no slot available wait until slot is available (can check every step as it should be quick)

[SimonPiCarter]

To make it work properly we need to forbid building too close to resource node