ENH: Output precision type is added to BRAINSTransformConvert

A new flags are added to BRAINSTransformConvert tool:
--outputPrecisionType

This flag can be set as "float" or "double", and it can work well with
other options like outputTransformType.

This patch is tested and works properly based on the last version of ITK
that includes updated transform file reader/writer filters.
The updated filters can convert their input transforms to the requested
output precision type automatically.