Analytical_Engine: Subgraphs method fails for directed graphs

#### Description:
Due to the way the traversal method starts from randomly selected entities it is possible that a directed graph cannot be traversed.
Currently the method will store the traversed parts as a sub graph.
For example this should return a single graph.

#### Steps to reproduce:
Use the test file - it may ne necessary to recompute the definition to see the error as the process randomly selects a start entity from which traversal initiates.

#### Expected behaviour:
A single graph should be the result

#### Test file(s):
https://burohappold.sharepoint.com/:u:/r/sites/BHoM/02_Current/12_Scripts/01_Issue/BHoM/BHoM_Engine/Analytical_Engine/%232271_subgraphtest.gh?csf=1&web=1&e=FaZ0K6