MRG: allow more ways to specify data dtype

A small refactoring to allow this:

    hdr = nib.Nifti1Header()
    hdr.set_data_dtype(float)

and this:

    hdr.set_data_dtype('f8')

Previously these would both raise an error.
Ahh my bad, didn't notice the initial indent. Thought they were functions not methods of a class. Pardon the noise....