STY: Use triple double-quoted string in docstrings (PEP-257)

[jond01]

''' -> """

• Some style in dicom/derivations

[codecov]

Codecov Report

Merging #918 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #918   +/-   ##
=======================================
  Coverage   91.73%   91.73%           
=======================================
  Files          97       97           
  Lines       12357    12357           
  Branches     2177     2177           
=======================================
  Hits        11336    11336           
  Misses        684      684           
  Partials      337      337           

Impacted Files	Coverage Δ
nibabel/cifti2/parse_cifti2.py	83.86% <ø> (ø)
nibabel/data.py	86.99% <ø> (ø)
nibabel/dataobj_images.py	93.75% <ø> (ø)
nibabel/ecat.py	88.07% <ø> (ø)
nibabel/loadsave.py	92.59% <ø> (ø)
nibabel/minc1.py	90.64% <ø> (ø)
nibabel/minc2.py	89.87% <ø> (ø)
nibabel/nicom/csareader.py	87.40% <ø> (ø)
nibabel/nicom/dicomreaders.py	58.76% <ø> (ø)
nibabel/nicom/dicomwrappers.py	90.90% <ø> (ø)
... and 28 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 966a01e...29ad025. Read the comment docs.

[effigies]

LGTM. One minor correction that I'll make myself.