Add initial implementation of BatteryModel with tests and project setup

tests/test_battery.py

@@ -0,0 +1,12 @@
+from batterymodel import BatteryModel
+
+def test_battery_stats():
+    assert BatteryModel()._max_charge_rate == 2000000 # Watts
+    assert BatteryModel()._max_discharge_rate == 2000000 # Watts
+    assert BatteryModel()._max_storage_volume == 4000000 # Wh
+    assert BatteryModel()._charging_efficiency == 0.05
+    assert BatteryModel()._discharging_efficiency == 0.05
+    assert BatteryModel()._lifetime_years == 10
+    assert BatteryModel()._lifetime_cycles == 50000
+    assert BatteryModel()._storage_volume_degradation_rate == 0.001 # %/cycle
+    assert BatteryModel()._fixed_operational_costs == 50000 # £/year