change shuttertime dtype to uint64 to allow for higher values

src/tempus/Tempus.py

@@ -370,7 +370,7 @@ class Tempus(Device):
         defaultValue=False,
         allowedStates={State.INIT, State.ON})
 
-    shutterTime = UInt16(
+    shutterTime = UInt64(
         displayedName="Shutter Time",
         unitSymbol=Unit.SECOND,
         metricPrefixSymbol=MetricPrefix.MILLI,