ENH: Warning when reading a PNG with UNKNOWN unit and non-unit spacing.

When using ITK's libpng warn when the read png file is not going to be
compatible with newer libpng libraries with regard to spacing.

ITK now correctly outputs PNG's with sCAL unit METER per commit:

  bd945330746905c5e813275690477ff8981e7b

The png files previously written by ITK have an UNKNOWN sCal field. With PNG
1.5, the value is ignored unlike the prior versions.

If a PNG is read with UNKNOWN unit but not unit spacing, suggest re-saving the
file so the spacing is not ignored.

An example with SimpleITK:

import SimpleITK as sitk
from glob import glob
fn = glob("*/*.png")
for f in fn:
    img = sitk.ReadImage(f)
    sitk.WriteImage(img, f)

Co-authored-by: Bradley Lowekamp <blowekamp@mail.nih.gov>

Change-Id: Ia4cb10d7144042cd3cc4df9714b6af275eb09272
STYLE: Put newline before long doxygen reference

Change-Id: I4da0867d671eddb05dc826036efe982d131ec1a5
Merge topic 'PNG-sCAL-warning-3'

d0901e44 ENH: Warning when reading a PNG with UNKNOWN unit and non-unit spacing.
Merge topic 'fix-doxygen-margin'

47fd12da STYLE: Put newline before long doxygen reference