[Neuroimaging] Nibabel API change - always read as float

[Brendan Moloney]

> I think the dtype argument is OK, it may be better than `asfloat`. It
> starts becoming a little complicated having to deal with all possible
> output types - for example rounding float to ints is not as
> straightforward as it may seem (for example you have to clip the
> output so as not to overflow the ints).

I think it is fine, maybe even preferable, to raise an exception is these
situations. Even then I agree that it complicates things.

-Brendan