code: public static ConfigProperties ofFlatMap(String key, Map<String, String> map) {
  
      SimpleConfigProperties root = new SimpleConfigProperties(key);
      root.fromFlatMap(map);
      return root;
    }
docs: Converts a flat {@link Map} of configuration values to hierarchical {@link ConfigProperties}. E.g. the flat map
  <code>{"foo.bar.some"="some-value", "foo.bar.other"="other-value"}</code> would result in {@link ConfigProperties}
  {@code myRoot} such that
  <code>myRoot.{@link #getChild(String...) getChild}("foo", "bar").{@link #getChildKeys()}</code> returns the
  {@link Collection} {"some", "other"} and
  <code>myRoot.{@link #getChildValue(String) getValue}("foo.bar.some")</code> returns "my-value".
  
  @param key the top-level key of the returned root {@link ConfigProperties}-node. Typically the empty string ("")
  for root.
  @param map the flat {@link Map} of the configuration values.
  @return the root {@link ConfigProperties}-node of the given flat {@link Map} converted to hierarchical
  {@link ConfigProperties}.