Add help string for entities returned by the API

Closes #16

src/main/java/eosc/eu/model/StorageElement.java

@@ -51,10 +51,14 @@ public class StorageElement extends StorageElementBase {
     public String downloadUrl;
 
     @JsonInclude(JsonInclude.Include.NON_EMPTY)
+    @Schema(description="File checksum in the form `algorithm:value`")
     public String checksum;
 
     @JsonInclude(JsonInclude.Include.NON_EMPTY)
-    public String checksumType;
+    @Schema(description="Can be used to group files in file sets (collections). This only " +
+                        "allows one level of grouping, for a deeper hierarchical structure " +
+                        "use the field 'path'.")
+    public String collection;
 
 
     /**

src/main/java/eosc/eu/model/StorageElementBase.java

@@ -2,6 +2,7 @@
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonInclude;
+import org.eclipse.microprofile.openapi.annotations.media.Schema;
 
 
 /**
@@ -13,8 +14,21 @@ public class StorageElementBase {
     public String kind;
 
     @JsonInclude(JsonInclude.Include.NON_EMPTY)
+    @Schema(description="The name of the file. Can be used, together with the field 'path', " +
+                        "to construct the destination path when this file is included in a " +
+                        "data transfer.")
     public String name;
 
+    /***
+     * The path of the file. Can be used, together with the field "name",
+     * to construct the destination path when this file is included in a
+     * data transfer.
+     */
+    @Schema(description="The path of the file in the data repository. Can be used, together with the field 'path', " +
+                        "to construct the destination path when this file is included in a " +
+                        "data transfer.")
+    public String path = "/";
+
 
     /**
      * Constructor