Overload class constructors with empty md (#159)
iso object can be initialized as empty (#159)
Added support for gmd:CI_Citation/gmd:identifier/gmd:RS_Identifier needed by INSPIRE
Added doctest for #159
Checking for unset elements (#159)
Merge pull request #161 from kalxas/159-jinja

Refactoring of iso.py so that MD_Metadata class can be used as an editor model