Changed interpreter representation of containers.

OLD:

    >>> Container("c'8 d'8 e'8")
    {c'8, d'8, e'8}

NEW:

    >>> Container("c'8 d'8 e'8")
    Container("c'8 d'8 e'8")

The same is now true for Cluster and GraceContainer.

Container initializer now also accepts an is_simultaneous=None keyword.
Merge branch 'master' of https://github.com/Abjad/abjad
Changed interpreter representation of contexts.

OLD:

    >>> Voice("c'8 d'8 e'8")
    Voice{3}

NEW:

    >>> Voice("c'8 d'8 e'8")
    Voice("c'8 d'8 e'8")

But note that this is still the case:

    >>> voice_1 = Voice("c'8 d'8 e'8")
    >>> voice_2 = Voice("f'8 g'8")
    >>> Staff([voice_1, voice_2])
    Staff<<2>>

The principle is that contexts now examine their contents before interpreter
representation creation: if all contents are leaves then contexts returns an
evaluable representation; otherwise contexts return the old-style abbreviated
(and nonevaluable) representation.

Doesn't need to be noted in the change log.