Support setting command doc_in and doc_out from docstrings

Yury Matveevrequested to merge
637-add-support-for-making-docstrings-for-commands-available into develop

Closes #637 (closed)

Since there is no general cmd doc, I propose, that we can parse docstring for arg and return info and feed it into in_type_desc and out_type_desc, respectively

The easies way to do it - to use external lib https://github.com/rr-/docstring_parser.

However, I made the code to be robust in case this dependency is not installed (eg for those who want to make a Deb package and do not want to backport docstring_parser)

Edited by Yury Matveev

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