Keywords were extracted via 3 different elements
`keyword-inspire-theme`, `keyword-controlled-other` and
`keyword-free-text`. The latter didn't actually do anything and the
second duplicated xpaths from the first and added a non-standard one.
A new `keywords` key has been added which contains all keyword objects,
including type. This is not used to modify the `tags` key right now.
Changes in multiplicity to support the ISO 19115 spec rather than just
the Gemini 2 one. Thanks to @dread for his help on this.
Summary of the changes:
* dataset-reference-date: Set to 1..*
Note that there was a bug with mutliple values allowed per date.
Returned object should now be like:
"dataset-reference-date": [{"type": "creation", "value": "2004-02"},
{"type": "revision", "value": "2006-07-03"}]
* metadata-languge: Set to 0..1
* resource-type: Set to *. That means that a list is now returned
* bbox: Set to *. Note that bboxes are now returned as objects such as:
[{"north":xxx, "south":xxx,, "east":xxx, "west":xxx}, {"north":xxx,
"south":xxx,, "east":xxx, "west":xxx}]
The existing Gemini based harvesters and validators have been adapted,
all tests pass.