Feature/allow csw constraints

[knudmoeller]

• extends ckanext.spatial.lib.CswService.getidentifiers() with constraints parameter, which will simply be passed on as the constraints parameter to owslib's get_records2()
• extends ckanext.spatial.harvesters.CSWHarvester with new get_constraints() function
  • CSWHarvester simply returns an empty list (i.e., no constraint), but sub-classes can overwrite this
  • get_constraints() is being called from gather_stage()
• Note: the current setup means that constraints can only be set if a sub-class of CSWHarvester is being used. This is exactly what I need. If constraints need to be used from plain CSWHarvester, additional code would be needed to allow entering the constraints as part of the harvester's config.