Replace constant strings with constant vals

src/main/kotlin/DocumentsSearchDocBuilder.kt

@@ -19,6 +19,7 @@
 package org.memobase
 
 import ch.memobase.rdf.NS
+import ch.memobase.rdf.RICO
 import com.beust.klaxon.JsonObject
 import org.apache.logging.log4j.LogManager
 import org.memobase.builders.AgentContainerBuilder
@@ -56,9 +57,9 @@ class DocumentsSearchDocBuilder(
 
         val record = input[JsonUtility.recordTag] ?: throw InvalidInputException("No record defined in the message.")
         val digitalObject =
-            input.values.firstOrNull { it["@type"] == NS.rico + "Instantiation" && it["type"] == "digitalObject" }
+            input.values.firstOrNull { it[Constants.atType] == RICO.Instantiation.uri && it[RICO.type.localName] == RICO.Types.Instantiation.digitalObject }
         val physicalObject =
-            input.values.firstOrNull { it["@type"] == NS.rico + "Instantiation" && it["type"] == "physicalObject" }
+            input.values.firstOrNull { it[Constants.atType] == RICO.Instantiation.uri && it[RICO.type.localName] == RICO.Types.Instantiation.physicalObject }
 
         val keywordIds = Extract.identifiers(record[Constants.hasSubject])
         val genreIds = Extract.identifiers(record[Constants.hasGenre])