Add edges field, refactor for testing

[kshefchek]

Adds curie prefix and count of edges to SimpleLoader (search and autocomplete). Prefix mappings are useful for filtering in addition to category. Edge count may be useful in faceted/scoped autocomplete to adjust rankings.

[kshefchek]

On prefix indexing, we can get the same functionality with the filter query syntax, for example,

fq=id:HP*

But this has some downsides, for example: rdf, rdfs CL, CLO

Therefore we need to remember to include a colon:

fq=id:HP\:*

The explicit index does not add much overhead in terms of space and indexing time (see http://ci.monarchinitiative.org/view/dev/job/create-and-load-search-core-from-scigraph-dev/ for a test run).

[putmantime]

+1