[unit system] add"um" alias

tomoscan/unitsystem/metricsystem.py

@@ -105,7 +105,7 @@ class MetricSystem(_Enum):
             return MetricSystem.CENTIMETER
         elif value.lower() in ('mm', 'millimeter'):
             return MetricSystem.MILLIMETER
-        elif value.lower() in ('micrometer'):
+        elif value.lower() in ('um', 'micrometer'):
             return MetricSystem.MICROMETER
         elif value.lower() in ('nm', 'nanometer'):
             return MetricSystem.NANOMETER