Added check for multiple identifiers in same context
Merge branch 'master' of https://github.com/INCF/nineml into population_selection
adding PopulationSelection element
Merge branch 'updates_to_spec' into population_selection
added examples directory
moved sample_xml_files out of catalog and into examples directory
added examples
added (rudimentary) unit-tests for component, population, and
populationselection.
Merge pull request #54 from tclose/population_selection

Added support for "PopulationSelection"
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