Add missing docstrings for classes/functions in data_utils

[oruebel]

Motivation

Improve documentation of data_utils by adding missing docstrings

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes problem and the solution?
• Is your contribution compliant with our coding style ? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using #XXX notation where XXX is the issue number ? By including "Fix #XXX" you allow GitHub to close the corresponding issue.

[codecov]

Codecov Report

Merging #149 into dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              dev     #149   +/-   ##
=======================================
  Coverage   69.75%   69.75%           
=======================================
  Files          29       29           
  Lines        5736     5736           
  Branches     1352     1352           
=======================================
  Hits         4001     4001           
  Misses       1307     1307           
  Partials      428      428

Impacted Files	Coverage Δ
src/hdmf/data_utils.py	83.7% <100%> (ø)	⬆️

---

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 9b6848f...776a577. Read the comment docs.