Removed try/except block in Quantity.dimensionality

As the Quantity class has an expensive __getattr__ this check was having
a measurable performance hit.

Close #251

docs/systems.rst

@@ -0,0 +1,56 @@
+.. systems_:
+
+Different Unit Systems (and default units)
+==========================================
+
+Pint Unit Registry has the concept of system, which is a group of units
+
+    >>> import pint
+    >>> ureg = pint.UnitRegistry(system='mks')
+    >>> ureg.system
+    'mks'
+
+This has an effect in the base units. For example:
+
+    >>> q = 3600. * ureg.meter / ureg.hour
+    >>> q.to_base_units()
+    <Quantity(1.0, 'meter / second')>
+
+We can take a look for the available systems
+
+    >>> ureg.systems
+    frozenset({'US', 'mks', 'imperial', 'cgs'})
+
+But if you change to cgs:
+
+    >>> ureg.system = 'cgs'
+    >>> q.to_base_units()
+    <Quantity(100.0, 'centimeter / second')>
+
+or more drastically to:
+
+    >>> ureg.system = 'imperial'
+    >>> '{:.3f}'.format(q.to_base_units())
+    '1.094 yard / second'
+
+..warning: In versions previous to 0.7 `to_base_units` returns quantities in the
+           units of the definition files (which are called root units). For the definition file
+           bundled with pint this is meter/gram/second. To get back this behaviour use `to_root_units`,
+           set `ureg.system = None`
+
+
+You can also use system to narrow down the list of compatible units:
+
+    >>> ureg.system = 'mks'
+    >>> ureg.get_compatible_units('meter')
+    frozenset({<Unit('light_year')>, <Unit('angstrom')>, <Unit('US_survey_mile')>, <Unit('yard')>, <Unit('US_survey_foot')>, <Unit('US_survey_yard')>, <Unit('inch')>, <Unit('rod')>, <Unit('mile')>, <Unit('barleycorn')>, <Unit('foot')>, <Unit('mil')>})
+
+
+    >>> ureg.system = 'imperial'
+    >>> ureg.get_compatible_units('meter')
+    frozenset({<Unit('US_survey_mile')>, <Unit('angstrom')>, <Unit('inch')>, <Unit('light_year')>, <Unit('barleycorn')>, <Unit('mile')>, <Unit('US_survey_foot')>, <Unit('rod')>, <Unit('US_survey_yard')>, <Unit('yard')>, <Unit('mil')>, <Unit('foot')>})
+
+    >>> ureg.imperial.pint
+    bla
+
+    >>> ureg.us.pint

pint/quantity.py

@@ -198,13 +198,13 @@ class _Quantity(SharedRegistryObject):
 
         return not bool(tmp.dimensionality)
 
+    _dimensionality = None
+
     @property
     def dimensionality(self):
         """Quantity's dimensionality (e.g. {length: 1, time: -1})
         """
-        try:
-            return self._dimensionality
-        except AttributeError:
+        if self._dimensionality is None:
             self._dimensionality = self._REGISTRY._get_dimensionality(self._units)
 
         return self._dimensionality