ENH: Add RegularStep gradient optimizer to ITKv4

We probably do not need this optimizer technically; however, having such
an optimizer is the preliminary step to convert "ImageRegistration1"
example to ITKv4.

Using ordinary GradientDescentOptimizerv4 does not provide convergence
for the example 1. Also, the results generated by other v4 optimizers
(e.g. GradientDescentLineSearchOptimizerv4) show a very fast
convergence, and they are not suitable for graphical interpretation of
the registration process that is necessary for the educational purposes.

Change-Id: I547f58f776952aebaa7d36ac2dd1540406617268
Merge topic 'changeRegExmplsToItkv4_5'

a21dd3f1 ENH: Add RegularStep gradient optimizer to ITKv4