ENH: add some tests for antsRegistration in float mode

Recently, antsRegistration is enhanced to be able to process data in single
precision. The related changes were pushed to ANTs repository, but there
were no test related to them.
The problem was that new tests could not be feeded by double type initial
transforms.
I converted those double type initial transforms to single precision
type using the newly enhanced BRAINSTransformConvert tool.
Then, I added some tests for the antsRegistration in single precision
mode. Test changes and related baselines are commited here.