ENH: Run 1 level of linear registrations in and make minStepLength back

3 changes are made in this new patch:

1- linear registration stages are run in 1 level of resolution with
shrinking factor of 1 and smoothing sigma of 0

2- A flag is added as
"initializeRegistrationByCurrentGenericTransform" that changes the
default behaviour of warping moving image before it is passed to the
BSpline registration filter.

3- "minimumStepLength" is added back to the BRAINSFit
Merge pull request #92 from aghayoor/addMinStepLengthToBFit

Add min step length to b fit