Resolve "Add the scripts into the package"

Closes #217 (closed)

The scripts run_constraints.py and show_model.py were moved but no entry point has been provided for them neither they were integrated to the run_model.py. This can be done in a separate issue.

The scripts run_model.py and run_session.py still can be called in the old style, like python src/virtmat/language/cli/run_model.py but this is not recommended because it uses the code in the git repository rather than those in the installed package.