Replace os.system with subprocess.run for plantuml calls

[mircealungu]

From review of PR #81.

In src/python/src/cli_interface.py, plantuml is invoked via os.system():

os.system(f"{python_executable} -m plantuml --server {plantuml_server}  {file_name}")

subprocess.run() would be safer (no shell interpretation) and provides better error handling. Note that subprocess is already imported but unused.