Adding regular step gradient descent magnitude tolerance

Change-Id: I73d7be8c48c68783a6121aaca75a381fac9a30b4
Adding GradientDescentLineSearch optimizer

Change-Id: I841377d58af0f34f3e0ae90e61f1855bda2c5857
FIXUP for ConjugateGradientLineSearch

Wrong enum type for optimizer factory was used.

Change-Id: If565b792a65227165e84d9bc4973a73b7cecf5ed
Update ITK registration branch with more optimizers

Change-Id: Ib4fe28f11604777b666d8ebbf78b1f9511041d9e
Update StopDescription after execution

Change-Id: I586d8d3d6f05ef6bced89661321de6dd907c53c3