Extend documentation of historical.process()

[francescolovat]

When trying to retrieve data from T000.csv, it was not fully clear how to do so in an efficient manner. When reading historical.process() docu, information about that function was missing and what was exactly the format of its argument id.

This PR extends its docstring and add example of id argument.

[codecov[bot]]

Codecov Report

Merging #63 (72e7fe7) into master (cbf4e1b) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #63   +/-   ##
=======================================
  Coverage   52.74%   52.74%           
=======================================
  Files          38       38           
  Lines        1583     1583           
=======================================
  Hits          835      835           
  Misses        748      748           

Impacted Files	Coverage Δ
item/historical/__init__.py	92.47% <100.00%> (ø)

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[khaeru]

Thanks @francescolovat!

The check failures here are unrelated to your PR, so I will make another to fix them and then ask you to rebase.

[khaeru]

64 is merged, so please rebase.

[francescolovat]

64 is merged, so please rebase.

@khaeru thanks. Ready to merge once CIs will pass..

[khaeru]

The check failures here are because this a PR from a fork which has no access to the secret key used to upload data to our Google Cloud Storage bucket. I will open a separate issue to disable this.