Slurm and QResources updates

[gpetretto]

This PR introduces the option to have an "empty" QRsources. Now process_placement can be None, representing a case of an empty set of resources, where no value should be set.

Also added all the possible states for SLURM to address issue #37.

[codecov]

Codecov Report

Attention: Patch coverage is 96.87500% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 63.16%. Comparing base (c722837) to head (2544441).

❗ Current head 2544441 differs from pull request most recent head 3ffe3ac. Consider uploading reports for the commit 3ffe3ac to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #38      +/-   ##
===========================================
+ Coverage    62.39%   63.16%   +0.76%     
===========================================
  Files           15       15              
  Lines         1117     1143      +26     
  Branches       216      222       +6     
===========================================
+ Hits           697      722      +25     
  Misses         418      418              
- Partials         2        3       +1     

Files	Coverage Δ
src/qtoolkit/core/data_objects.py	100.00% <100.00%> (ø)
src/qtoolkit/io/shell.py	100.00% <100.00%> (ø)
src/qtoolkit/io/slurm.py	78.31% <100.00%> (+1.00%)	⬆️
src/qtoolkit/io/base.py	82.60% <83.33%> (-0.73%)	⬇️

[davidwaroquiers]

Perfect! Just a few typos (missing ".") and maybe add a test for _convert_qresources of shellio ? (+ also one question regarding the slurm states mapping, if you are clear about it, it's fine to me, otherwise, maybe put some comment about these two slurm states)