Added inspect(note).get_grace_container() method.

inspect(note).get_grace_container() goes with
inspect(note).get_grace_containers().

These pairs of inspect methods function in parallel:

    inspect(note).get_indicators()
    inspect(note).get_indicator()

    inspect(note).get_grace_containers()
    inspect(note).get_grace_container()

    inspect(note).get_spanners()
    inspect(note).get_spanner()

* Use the plural when zero or more of something may attach to a component.

* Use the singular when you're certain only one of something attaches to a
  component.

* The singular is a convenience wrapper around the plural.