Adding explicit conversion constructor to transform from base class

This constructor currently does not change the underlying transform
type, it only tries to cast it to this derived specific interface. If
it's fails the base class methods are still functional, but the
specialized methods will throw and exception if called.

Change-Id: Ie37082d94838af8bc2377441165300be65b0eb93
Adding comprehensive testing for the versor initializer

Change-Id: I6faabe589165026d4f5badf9bda1e1c17dc52380
Explicitly use Self qualifier for virtual method in constructor

In the constructor if the Transform class a call to a the virtual
SetPimpleTransform method was done. However since the derived
class hasn't been initialized yet, only the current class's method is
called. Make this explicit.

Change-Id: Ibb9918559d53161173e626ad85d555c7cb0e19b9
Merge topic 'SIMPLEITK-563_WrapCenteredVersorTransformInitializer' into next

9c7c9e47 Explicitly use Self qualifier for virtual method in constructor
6e080a45 Adding comprehensive testing for the versor initializer
1654b27d Adding explicit conversion constructor to transform from base class