add feature: add subprocess element support (continued)

[nitram509]

Motivation/Abstract

adding support for SubProcess elements in BPMN

fixes #246

Description/Comments

Based on the great work from #247 , this changes some aspects of the implementation.

• keeping the helper functions FindSequenceFlows() and FindFirstSequenceFlow() in their current place
• passing an activity object to handle process and subProcess (instead of just the state)

Checklist

Depending on your PR, please ensure the overall consistency/integrity of the project remains.
Please tick just one check item per section below

Tests

• did you update or create tests for your code changes?
• not relevant

Code examples

• did you update or add example code snippets, which relate to your code changes
• not relevant

Documentation

• did you update or create documentation, which relates to your code changes
• not relevant

[codecov]

Codecov Report

Attention: Patch coverage is 40.92827% with 140 lines in your changes missing coverage. Please review.

Project coverage is 63.76%. Comparing base (42caf7b) to head (da2f569).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/spec/BPMN20/elements.go	0.00%	68 Missing ⚠️
pkg/spec/BPMN20/helper.go	0.00%	43 Missing ⚠️
pkg/bpmn_engine/engine.go	83.58%	10 Missing and 1 partial ⚠️
pkg/bpmn_engine/marshalling.go	9.09%	10 Missing ⚠️
pkg/bpmn_engine/process_instance.go	55.55%	4 Missing ⚠️
pkg/bpmn_engine/sub_process.go	75.00%	2 Missing ⚠️
pkg/bpmn_engine/timer.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #264      +/-   ##
==========================================
- Coverage   66.02%   63.76%   -2.26%     
==========================================
  Files          27       28       +1     
  Lines        1757     1896     +139     
==========================================
+ Hits         1160     1209      +49     
- Misses        553      642      +89     
- Partials       44       45       +1     

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