Add the option --version to verdi to display current version

[sphuber]

Fixes #1569 partially. The issue also requested the ability to detect where the current version is installed and if from a git repository which commit is checked out. I am not sure this is trivial to implement and should maybe be handled in a separate issue, as it would be good to already add this simplified version of the feature to develop.

[codecov-io]

Codecov Report

Merging #1811 into develop will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1811      +/-   ##
===========================================
+ Coverage    66.58%   66.61%   +0.02%     
===========================================
  Files          314      314              
  Lines        32346    32346              
===========================================
+ Hits         21538    21547       +9     
+ Misses       10808    10799       -9

Impacted Files	Coverage Δ
aiida/backends/djsite/db/models.py	76.23% <0%> (+0.88%)	⬆️
aiida/backends/djsite/globalsettings.py	86.84% <0%> (+5.26%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1dfd4a8...b3492b2. Read the comment docs.

[giovannipizzi]

Ok for me. If getting the git version is complicated, maybe at least adding also the aiida.__path__?