Add support for grouping elements

[yt-ms]

• [X] fix https://github.com/Midnighter/structurizr-python/issues/72
• [X] description of feature/fix
• [X] tests added/passed
• [X] add an entry to the next release

Structurizr now supports grouping elements to provide more structure (e.g. Containers within a System). This change adds a group property to such elements via a GroupableElement supertype, in line with the equivalent Java implementation.

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[codecov-commenter]

Codecov Report

Merging #75 (cbfd945) into devel (3646417) will increase coverage by 0.39%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            devel      #75      +/-   ##
==========================================
+ Coverage   91.18%   91.57%   +0.39%     
==========================================
  Files          69       70       +1     
  Lines        2109     2124      +15     
  Branches      210      210              
==========================================
+ Hits         1923     1945      +22     
+ Misses        167      160       -7     
  Partials       19       19              

Impacted Files	Coverage Δ
src/structurizr/model/groupable_element.py	100.00% <100.00%> (ø)
src/structurizr/model/static_structure_element.py	92.30% <100.00%> (+0.64%)	:arrow_up:
src/structurizr/view/automatic_layout.py	100.00% <0.00%> (+30.43%)	:arrow_up:

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