Docs fix

[mroberge]

Improve docstrings.

• add links to next pages
• fix typos, errors, and problems
• standardize module docstrings
• move to Google style docstrings
• put ipynb files into a notebooks folder.

[codecov-io]

Codecov Report

Merging #40 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #40   +/-   ##
========================================
  Coverage    74.21%   74.21%           
========================================
  Files            7        7           
  Lines          380      380           
  Branches        76       76           
========================================
  Hits           282      282           
  Misses          68       68           
  Partials        30       30

Impacted Files	Coverage Δ
hydrofunctions/hydrofunctions.py	74.52% <ø> (ø)	⬆️
hydrofunctions/exceptions.py	100% <ø> (ø)	⬆️
hydrofunctions/typing.py	81.48% <ø> (ø)	⬆️
hydrofunctions/station.py	86.95% <ø> (ø)	⬆️
hydrofunctions/helpers.py	100% <ø> (ø)	⬆️
hydrofunctions/charts.py	53.33% <ø> (ø)	⬆️
hydrofunctions/__init__.py	100% <ø> (ø)	⬆️

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