Add Base model type from Schwalbe project

function toMap(objs) {

    return new Map(objs.map(obj => [obj.id, obj]))

}

export class Base {

    /**

     * A Map mapping ids to instances of `Base`.

     * Must be treated as private.

     */

    static _byId = null

    /**

     * @return A list of all instances of `Base`

     */

    static get all() {

        return this._byId ? Array.from(this._byId.values()) : null

    }

    /**

     * @param {str} id A id

     * @returns The instance with the given id, if it exists

     */

    static get(id) {

        return this._byId ? this._byId.get(id) : null

    }

    /**

     * Load the objects from the API, unless the objects have already been loaded.

     * They are then stored in `this.byUid`.

     *

     * @returns Nothing.

     */

    static async load() {

        if (!this._byId) {

            const objects = await this.forceLoad()

            // Hack: We get plain JSON objects from the server, however, we'd like

            // them to actually be instances of the classes we define. So, we first create a new empty

            // instance of whatever subclass of `Base` we are currently in using `new this()`,

            // and then copy all properties from the plain object over to the instance

            // using `Object.assign`.

            const instances = objects.map(obj =>

                Object.assign(new this(), obj)

            )

            this._byId = toMap(instances)

        }

    }

    /**

     * If the objects were already loaded, forget them and load them again.

     * If the objects were not yet loaded, don't do anything.

     */

    static refresh() {

        if (this._byId) {

            this._byId = null

            this.load()

        }

    }

    /**

     * Load the objects from the API and return them.

     * Abstract method, must be overriden in child classes.

     *

     * @returns A list of instances.

     */

    static async forceLoad() {}

}