add sleep util

src/mol-util/sleep.ts

+/**
+ * Copyright (c) 2019 Mol* contributors, licensed under MIT, See LICENSE file for more info.
+ *
+ * @author Alexander Rose <alexander.rose@weirdbyte.de>
+ */
+
+export function sleep(milliseconds: number) {
+    return new Promise(resolve => setTimeout(resolve, milliseconds))
+}
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