carry data type forward

[iancharest]

testA.nii.gz (matlab) is much smaller because it is an int16. somehow the python code doesn't keep file types.

[kendrickkay]

the matlab version tries hard to preserve data class and also allows the user to control the data class if they want.

[kendrickkay]

If the data class is controllable in the Python side of things, this would help keep file sizes down