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"