WIP: CIFTI2 spec reader and writer

- [x] CIFTI2 reader
- [x] CIFTI2 writer
- [x] tests
- [x] integration with nifti2
Does nibabel have a default unit for space?  The purpose of these exponents is to reduce rounding while keeping the values it applies to easy to read in the XML, so that this detail could be easily hidden by readers/writers.  However, I see that your Nifti1Image seems to also just expose the xyz (and t) units, so perhaps this is consistent.