ENH: Convert savedState file to the restored forward and inverse composites

BRAINSTransformConvert reads the saved registration state file (in the
format of a composite transform. Then it restores SyN transform from its
internal state transforms and writes restored forward and inverse
composite transforms to the disk.

The usage:

~/BRAINSTransformConvert \
--inputTransform ./ANTsSavedState.h5 \
--outputTransformType Same \
--outputPrecisionType double \
--outputTransform outputPrefix

Then, it writes the forward and inverse composite transforms as
“outputPrefixComposite.h5” and “outputPrefixInverseComposite.h5” to the
disk.