Optimise vocab loading

[wu-lee]

Generating the vocabs gets very slow when ther are more than one, delaying the map loading for 30 seconds or longer. This can probably be sped up with more selective loading and/or caching.

[wu-lee]

It turns out this is just that vocab queries don't have the default graph uri set.

I compared the speed of get_vocabs.php to a command-line SPARQL query via curl (see seodo using the same query, and the latter was less than half a second. The critical difference was the default-graph-uri.

This info also is in the config/*/default-graph-uri.txt files of a map website, so in principle get_vocabs.php could get it from there, but there vocab queries are done once and currently shared by all datasets, so it isn't clear how to resolve that and pick the right one.

Therefore I decided it was a parameter like endpoint in config.json, which needs to be provided once per vocab query. (I've left it optional however, so existing sea-map sites will still work without it.)

[wu-lee]

I've also updated ica-project to use the new config.json setting. Will deploy this along with all the other ICA updates shortly.

[ColmMassey]

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