ENH:  Initializing the transform center.

Linear transforms derived from the itk::MatrixOffsetTransformBase
class default to a center of rotation at the origin.  However,
this is generally a suboptimal starting point for optimization.
Instead, we use the following heuristics (h/t to Brad Lowekamp
for pointing this out):
  1) we use the center of the previous linear transform
     (presumably optimized) to initialize the current linear
     transform.
  2) a special case would be if someone wanted to optimize a
     linear transform following optimization of a deformable
     transform.  What we do in this case is go back further
     in the queue to find the closest, previous linear transform
     and use its center.
  3) if there is no previous transforms, we leave the center of
     mass as the default.

Change-Id: Iacb0442e0db51a1d004e9028868ce36fe530341e
Merge topic 'InitializeTransformCenter'

c0f3cbb6 ENH:  Initializing the transform center.