OreSatConfig can be called with None/no argument

Most invocations of OreSatConfigs at this point are with
Mission.default() as the argument so we might as well be nice and do it
here instead of asking each user.

mypy got a little picky about types so this change ended up being bigger
than I expected but most of it is just mission -> self.mission.

oresat_configs/__init__.py

@@ -32,27 +32,31 @@
 class OreSatConfig:
     """All the configs for an OreSat mission."""
 
-    def __init__(self, mission: Union[Mission, str]):
+    def __init__(self, mission: Union[Mission, str, None] = None):
         """The parameter mission may be:
         - a string, either short or long mission name ('0', 'OreSat0.5', ...)
         - a Mission (ORESAT0, ...)
+        - Omitted or None, in which case Mission.default() is chosen
 
         It will be used to derive the appropriate Mission, the collection of
         constants associated with a specific oresat mission.
         """
-        if isinstance(mission, str):
-            mission = Mission.from_string(mission)
-        elif not isinstance(mission, Mission):
+        if mission is None:
+            self.mission = Mission.default()
+        elif isinstance(mission, str):
+            self.mission = Mission.from_string(mission)
+        elif isinstance(mission, Mission):
+            self.mission = mission
+        else:
             raise TypeError(f"Unsupported mission type: '{type(mission)}'")
 
-        self.mission = mission
-        with as_file(mission.beacon) as path:
+        with as_file(self.mission.beacon) as path:
             beacon_config = BeaconConfig.from_yaml(path)
-        with as_file(mission.cards) as path:
+        with as_file(self.mission.cards) as path:
             self.cards = cards_from_csv(path)
-        self.configs = _load_configs(self.cards, mission.overlays)
-        self.od_db = _gen_od_db(mission, self.cards, beacon_config, self.configs)
+        self.configs = _load_configs(self.cards, self.mission.overlays)
+        self.od_db = _gen_od_db(self.mission, self.cards, beacon_config, self.configs)
         c3_od = self.od_db["c3"]
         self.beacon_def = _gen_c3_beacon_defs(c3_od, beacon_config)
         self.fram_def = _gen_c3_fram_defs(c3_od, self.configs["c3"])
-        self.fw_base_od = _gen_fw_base_od(mission)
+        self.fw_base_od = _gen_fw_base_od(self.mission)