Improved docstrings and some type hints.

[TimothyCera-NOAA]

• Removed markdown markup in docstrings in favor of using "--docformat numpy" for the pdoc command in create_docs.sh.
• Used numpy format for the "Return" sections to be properly rendered by "pdoc --docformat numpy ...".
• Removed blank lines between parameters in the "Parameters" section.
• Added missing docstrings.
• Minor improvements to type hints:
  • np.array to NDArray,
  • Optional[] wherever argument could be None,
  • Addition of Union where needed.