Bug fix release for the 2.3 series.

Most work on NiBabel so far has been by Matthew Brett (MB), Michael Hanke (MH)
Ben Cipollini (BC), Marc-Alexandre Côté (MC), Chris Markiewicz (CM), Stephan
Gerhard (SG), Eric Larson (EL), Yaroslav Halchenko (YOH) and Chris Cheng (CC).

References like "pr/298" refer to github pull request numbers.

2.3.1 (Tuesday 16 October 2018)

New features

• nib-diff command line tool for comparing image files (pr/617, pr/672,
  pr/678) (CC, reviewed by YOH, Pradeep Raamana and CM)

Enhancements

• Speed up reading of numeric arrays in CIFTI2 (pr/655) (Michiel Cottaar,
  reviewed by CM)
• Add ndim property to ArrayProxy and DataobjImage (pr/674) (CM,
  reviewed by MB)

Bug fixes

• Deterministic deduction of slice ordering in degenerate cases (pr/647)
  (YOH, reviewed by CM)
• Allow 0ms TR in MGH files (pr/653) (EL, reviewed by CM)
• Allow for PPC64 little-endian long doubles (pr/658) (MB, reviewed by CM)
• Correct construction of FreeSurfer annotation labels (pr/666) (CM, reviewed
  by EL, Paul D. McCarthy)
• Fix logic for persisting filehandles with indexed-gzip (pr/679) (Paul D.
  McCarthy, reviewed by CM)

Maintenance

• Fix semantic error in coordinate systems documentation (pr/646) (Ariel
  Rokem, reviewed by CM, MB)
• Test on Python 3.7, minor associated fixes (pr/651) (CM, reviewed by Gregory
  R. Lee, MB)