Move more code into separate classes

It might make logical sense.  Honestly, I'm just tired of the code for
World being so long.

aoe/gather.py

@@ -1,6 +1,7 @@
 import copy
 
 from aoe.types import *
+from aoe.exceptions import *
 
 
 
@@ -136,6 +137,23 @@ class Gather():
     BIG_NUMBER = 1e6
 
 
+    def check_gather(self, world, task, count=1):
+        if task in [Tasks.IDLE, Tasks.BUILD]:
+            return
+
+        required = self.required_gather_buildings[task]
+        have = set(world.buildings)
+
+        match = any(map(have.issuperset, required))
+
+        if not match:
+            raise GatherBuildingNotAvailableException
+
+        if task == task.FARM:
+            if world.villager_tasks[task] + count > len(world.unused_farms):
+                raise GatherBuildingNotAvailableException
+
+
     def __init__(self):
         self.gather_rates = {
                 Tasks.BERRIES: 18.6/60,