Final code tweaks

[mattjshannon]

This PR contains the follow changes:

• Updated any unfinished docstrings (Google style)
• "data" folder is now labelled "resources", note this is where the pickle gets downloaded to.
• Converted unit tests to use pytest (simple fix)
• Added a flowchart and some text to README.md. We need to decide if a different image should be here; and in any case, writer.py no longer exists, so we can update the figure.

[codecov-io]

Codecov Report

Merging #28 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #28   +/-   ##
=======================================
  Coverage   81.61%   81.61%           
=======================================
  Files           4        4           
  Lines         359      359           
=======================================
  Hits          293      293           
  Misses         66       66

Impacted Files	Coverage Δ
pypahdb/decomposer.py	92.62% <ø> (ø)	:arrow_up:
pypahdb/decomposer_base.py	75.53% <100%> (ø)	:arrow_up:

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