Changed "reduce_op" to "operator" in the XML serialization of AnalogReducePort, in accordance with the spec. Internally, the Python API still uses "reduce_op" as the attribute name, probably we should change this.
InitialValue now subclasses Property
Quantities should handle both int and float (the spec states "real numbers")
Fixed a few things relating to PopulationSelection
Added built-in dimensions and units to Python library
Some fixes to the Python units implementation.
Merge pull request #56 from apdavison/master

Assorted minor fixes; added some built-in dimensions and units to Python library
Merge remote-tracking branch 'upstream/master' into random_distributions

Conflicts:
	lib9ml/python/nineml/abstraction_layer/dynamics/writers/xml_writer.py
	lib9ml/python/nineml/abstraction_layer/units.py