``Affine2DBase``, ``BlendedAffine2D`` and ``CompositeAffine2D``

In order to support 3-dimensional transforms, the following deprecations have been

made. The deprecated classes can be directly replaced with the new alternatives

- ``Affine2DBase`` has been replaced with ``AffineImmutable``

- ``BlendedAffine2D`` has been replaced with ``BlendedAffine``

- ``CompositeAffine2D`` has been replaced with ``CompositeAffine``.

Added support for Non 2-dimensional transforms

Support has been added for transforms in matplotlib that aren't 2D.

``AffineImmutable`` directly replaces ``Affine2DBase``, and introduces a ``dims``

keyword that specifies the dimension of the transform, defaulting to 2.

``BlendedAffine`` directly replaces ``BlendedAffine2D``, and can blend more than

two transforms, with each transform handling a different axis.

``CompositeAffine`` directly replaces ``CompositeAffine2D``, and composes two Affine

transforms, as long as they have the same dimensions.

``IdentityTransform`` can create identity matrices of any dimension, through the use of

the ``dims`` keyword.