render set: helper refactoring

[sbesson]

While working on a feature allowing to process multiple images with multiple rendering settings, I found it useful to refactor the logic of the render set command dealing with loading and parsing rendering settings from a source (object or file on disk).

The changes included in this PR should not changing behavior, include some cleanup (print ->debug) as well as unit tests starting to cover some of the individual functionalities

[snoopycrimecop]

Conflicting PR. Removed from build OMERO-plugins-push#101. See the console output for more details.
Possible conflicts:

• PR Parallelize the set command #33 dominikl 'Parallelize the set command'
  • src/omero_cli_render.py

--conflicts Conflict resolved in build OMERO-plugins-push#110. See the console output for more details.

[dominikl]

Looks good. Just one comment (see above), and when I ran it, got a warning:

/home/dominik/py3/lib/python3.6/site-packages/omero/util/pydict_text_io.py:81: YAMLLoadWarning: calling yaml.load_all() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
  data = list(yaml.load_all(rawdata))

Edit: The warning doesn't have anything to do with the PR though, seems to be a python3 thing.

[dominikl]

Tested again, works. Good to merge 👍