Provide metainfo units for import.

nomad/metainfo/__init__.py

@@ -293,4 +293,5 @@ from .metainfo import (
     SectionAnnotation,
     SectionProxy,
     derived,
-    constraint)
+    constraint,
+    units)

nomad/metainfo/metainfo.py

@@ -31,7 +31,7 @@ import pytz
 import docstring_parser
 import jmespath
 
-from nomad.units import ureg
+from nomad.units import ureg as units
 
 
 m_package: 'Package' = None
@@ -221,7 +221,7 @@ class _Dimension(DataType):
 class _Unit(DataType):
     def set_normalize(self, section, quantity_def: 'Quantity', value):
         if isinstance(value, str):
-            value = ureg.parse_units(value)
+            value = units.parse_units(value)
 
         elif not isinstance(value, pint.unit._Unit):
             raise TypeError('Units must be given as str or pint Unit instances.')
@@ -232,7 +232,7 @@ class _Unit(DataType):
         return value.__str__()
 
     def deserialize(self, section, quantity_def: 'Quantity', value):
-        return ureg.parse_units(value)
+        return units.parse_units(value)
 
 
 class _Callable(DataType):