BUG:  Need to explicitly specify spline order.

This patch addresses a couple issues:
1) The spline order needed to be explicitly specified
in the B-spline reconstruction.  Since most people use
the default spline order = 3, this won't have much of
an effect but it's a bug that needed to be fixed.
2) Since the phi lattice in the B-spline filter is
often used as an output, I formalized this by setting
it as this->GetOutput( 1 ).

I also rewrote parts of N4 to try to flush out an issue
we're having on certain linux machines which helps with
readability.

Change-Id: Ic06e6e2227e1675d08e5fb13cbacbbc208fd3600
BUG: Add GetInverse for IdentityTransform

This function is trivially implemented as an identity transform is
it's own inverse.

Change-Id: I301ca1e2b2584c0ab77c10b727eaccd4ca68a954
ENH: Adding SetIdentity method to base DisplacementField class

While missing from the Transform base class, most transform implement
a SetIdentity method. This method is useful to present a uniform
simple interface to ITK's transform class.

It set the displacement and the inverse displacement fields, if
provided, to zero vectors.

Change-Id: I2881a1c77456c985d1d4c01c10678c6ffa1fc47f
BUG: Uninitialized m_TransformDirection in ComplexToComplexFFTImageFilter.

Detected by valgrind.

Change-Id: I0506b17a392032fb80ca2e7d03f5eaa5c0a5ffac
Merge topic 'AddDisplacementFieldSetIdentity'

5df4fa99 ENH: Adding SetIdentity method to base DisplacementField class
Merge topic 'AddIdentityTransformGetInverse'

7cd6c6e0 BUG: Add GetInverse for IdentityTransform
Merge topic 'ComplexToComplexUninitialized'

5d6a1e04 BUG: Uninitialized m_TransformDirection in ComplexToComplexFFTImageFilter.
Merge topic 'MiscN4Fixes'

bd22bcf2 BUG:  Need to explicitly specify spline order.