split keywords by , in facet, then recalculate

[pvgenuchten]

Overview

This PR introduces a procedure which splits keywords by ',' and recalculates totals after db query
Similar mechanism could be used for other fields which are ,-separated
I see 2 alternatives for this PR:

• keywords not to be stored as concatenated string, but actual list
• introduce an in-db procedure to split the keywords-string as part of the facet query

Related Issue / Discussion

resolves #1065

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

[pvgenuchten]

we found an interesting side effect here (potentially related)

if a term contains a space, the aggregated statistics provide a much higher number then when you filter by that term

after filtering by 'water holding capacity'