NF: add nifti1 / nifti2 / analyze detection logic

Use early bytes from file to detect whether this file is nifti1, nifti2,
analyze, or neither.
RF: add nifti2 names to top level namespace
RF: use nifti 1 / 2 detection in save / load

Special-case nifti 1 / 2 pair saving.

Extend tests for single <-> pair conversion.

Allow '.hdr' as extension for saving to pair.
NF: add nifti2 example data files
WIP: making nifti2 example file tests work