[MetricSystem] add __str__ function

tomoscan/unitsystem/metricsystem.py

@@ -124,6 +124,28 @@ class MetricSystem(_Enum):
         else:
             raise ValueError("Cannot convert: %s" % value)
 
+    def __str__(self):
+        if self == MetricSystem.METER:
+            return "m"
+        elif self == MetricSystem.CENTIMETER:
+            return "cm"
+        elif self == MetricSystem.MILLIMETER:
+            return "mm"
+        elif self == MetricSystem.MICROMETER:
+            return "um"
+        elif self == MetricSystem.NANOMETER:
+            return "nm"
+        elif self == MetricSystem.KILOELECTRONVOLT:
+            return "keV"
+        elif self == MetricSystem.ELECTRONVOLT:
+            return "eV"
+        elif self == MetricSystem.JOULE:
+            return "J"
+        elif self == MetricSystem.KILOJOULE:
+            return "kJ"
+        else:
+            raise ValueError("Cannot convert: %s" % value)
+
 
 m = MetricSystem.METER
 meter = MetricSystem.METER