Separate condition check and graph construct call in ModelRead

[dlamoris]

Similar to #100, the gsp ModelRead should check conditions and get target graph first and use the concrete graph uri to get the model.
https://github.com/Open-MBEE/flexo-mms-layer1-service/blob/develop/src/main/kotlin/org/openmbee/flexo/mms/routes/gsp/ModelRead.kt

The result should be similar to what ModelQuery and LockQuery does https://github.com/Open-MBEE/flexo-mms-layer1-service/blob/develop/src/main/kotlin/org/openmbee/flexo/mms/UserQuery.kt#L27

According to the spec, the underlying graph store, if there's a gsp endpoint, should allow a GET call instead of needing a construct query https://www.w3.org/TR/sparql11-http-rdf-update/#http-get

The following two operations are considered to be equivalent

GET /rdf-graph-store?graph=..graph_uri.. HTTP/1.1
Host: example.com
Accept: text/turtle; charset=utf-8

CONSTRUCT { ?s ?p ?o } WHERE { GRAPH <graph_uri> { ?s ?p ?o } }

This would allow us to use the GET directly instead of the construct query, though not sure if there's any performance benefit, depending on the backend