added LEMS formatted units and dimensions
convert units/dimension strings loaded from xml attributes into
Unit/Dimension objects
added unit dimensions to catalog files
Merge pull request #55 from tclose/lems_dimensions

Lems dimensions
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
Added equals magic method for Unit and Dimension
Added check for matching dimensions
debugged dimension checking on properties and added unit test
added missing file for unit test
added uncertml examples to repository
removed comments
imported built-in random distributions from uncertml
moved parameters above RandomDistribution tag in randomdistribution
catalog of built-in random distributions
added missing parameters to binomial distributions