ENH: Add functions for File entities

For https://gitlab.indiscale.com/caosdb/customers/cfaed/management/-/issues/29. This introduce a specific treatment of file entities and some other helper functions. This MR also introduces the retrieval with multiple ids.

Test together with f-inline-references branch of complexity wiki (use server profile in local-inline-references). npm pack this repo, then npm install it in the complexitywiki and start the react app. Verify that the functionality introduced here (files, Property.isList, and multi-ID retrieve) work as expected.

src/Entity.js

@@ -39,7 +39,7 @@ function _getRoleString(role) {
     case roles.ENTITY_ROLE_FILE:
         return "FILE";
     default:
-        throw `Unknown role ${role}.`;
+        throw new Error(`Unknown role ${role}.`);
     }
 }