lp:~widelands-dev/widelands/ai_blocked_fields_tweaks

Few changes in regard to blocked_fields list - it contains fields that are temporarily blocked for construction of new buildings, usually due to space consumer nearby or impossibility to connect a region to road network with warehouse.

Changes are:
- AI is more accurate to calculate fields that are not accessible, it uses find_reachable_fields to identify such region. And then block it first for 2 minutes and than for 10 minutes
- When calculating available spots it deducts blocked fields, this can make a difference especially if AI is in shortage of buildable fields