Add get all metadata with certain field method

I refactored the count query into a separate method so it can be used by
multiple methods. Then I added a method to return all records which have
a certain field defined. A concrete value can be passed in addition.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Group accessibility results per source portal

For GovData.de the accessibility results can be grouped by the
meta-metadata field original_metadata_portal. I am retrieving all
records which have this field set and then average the metric score
mapped to the portal.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Visualize accessibility metrics grouped by portals

For GovData.de I have added a visualization for the average score of the
accessibility metric results grouped by the portals providing the data.
For that I have used circle packing of d3.js. I believe, however, that a
simple bar chart will illustrate the results much better.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>