ENH:  Accessing optimizer weights from the methods.

The v4 optimizers have a m_Weights variable which
permits applying prior knowledge to the update
parameters.  This allows, for example, the user
to limit a displacement field transform in a
specified (e.g. only in the x-y plane).  This
patch makes this functionality accessible to the
user from the registration methods.

Change-Id: Ie6fc1b571d12fc988091fe6f18b24b5ecd275d52
Merge topic 'OptimizerWeightsAccess'

00c2c36 ENH:  Accessing optimizer weights from the methods.