store FES filter as dict key to allow for repository parsing

[tomkralidis]

Overview

This PR introduces the ability to store the OGC filter as a generic dict as part of the constraint which is passed to repository queries. This is an interim step to which the ultimate goal is to have pycsw repository objects delegated the responsibility to parse OGC filter grammar into their specific syntax.

Longer term, default/internal pycsw refactoring will occur around pyfes. This PR immediately enables any repository plugin to tie filters deep into their backend by allowing translation functionality against a Python friendly object/representation of OGC filter grammar.

Sample OGC filter parsed into a dict:

{
    "ogc:Filter": {
        "ogc:PropertyIsLike": {
            "@wildCard": "%",
            "@singleChar": "_",
            "@escapeChar": "\\",
            "ogc:PropertyName": "csw:AnyText",
            "ogc:Literal": "foo"
         }
     }
}

Related Issue / Discussion

• SOLR backend through pysolr #208
• add support for Elasticsearch #410
• add support for CKAN integration #95

Additional Information

Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines

[tomkralidis]

Note: simple example implementation in https://gist.github.com/tomkralidis/6919d32da01b62b5b4e76b9254751e9b