Using CSW harvester OutputSchema is ignored while gmd is imposed

[ccancellieri]

Hi,
CSW is a very flexible and powerful standard which allows clients to query a catalog in several ways.
Normally a getCapabilities request is performed by the client to understand which formats are available etc.
The OWLlib is a good library but it still lack lots of low level functionalities like this (geopython/OWSLib#605).
I'm starting a discussion to introduce changes to that library but in the meanwhile I don't see why we can't have this here as well.
F.e. we could use the CSWharvester to gather metadata with different output schema formats at the moment gmd is imposed!!!

I'm going to propose a simple pull request (tested) which allows us to specify the target schema from the harvester configuration so if the server supports that schema it would be possible to obtain metadata in different formats.

My usecase is:

1. I'd like to query GeoNetwork with csw asking for iso19115* specifying the outputschema
2. I would like to specify it into the configuration (per harvester configuration).

With the PR I'm going to propose it would be possible.