Make pumps mutually exclusive

Forces only one pump to be active at a time, to avoid the sudden inrush current browning out the Pi or causing weirdness when multiple pumps fire and drop the voltage.

library/tests/test_lock.py

@@ -0,0 +1,37 @@
+import time
+
+
+def test_pumps_are_mutually_exclusive(GPIO, smbus):
+    from grow.pump import Pump, global_lock
+
+    ch1 = Pump(channel=1)
+    ch2 = Pump(channel=2)
+    ch3 = Pump(channel=3)
+
+    ch1.dose(speed=0.5, timeout=1.0, blocking=False)
+
+    assert global_lock.locked() is True
+
+    assert ch2.dose(speed=0.5) is False
+    assert ch2.dose(speed=0.5, blocking=False) is False
+
+    assert ch3.dose(speed=0.5) is False
+    assert ch3.dose(speed=0.5, blocking=False) is False
+
+
+def test_pumps_run_sequentially(GPIO, smbus):
+    from grow.pump import Pump, global_lock
+
+    ch1 = Pump(channel=1)
+    ch2 = Pump(channel=2)
+    ch3 = Pump(channel=3)
+
+    assert ch1.dose(speed=0.5, timeout=0.1, blocking=False) is True
+    assert global_lock.locked() is True
+    time.sleep(0.3)
+    assert ch2.dose(speed=0.5, timeout=0.1, blocking=False) is True
+    assert global_lock.locked() is True
+    time.sleep(0.3)
+    assert ch3.dose(speed=0.5, timeout=0.1, blocking=False) is True
+    assert global_lock.locked() is True
+    time.sleep(0.3)