Reworked app.run exit code (#73)

* allow run to return exit_code in non-interactive mode
* add tests for exit code from run.  Add variable substituion test for exec
* remove unneeded apptainer def
* bump version
* validate the substitution in test_exec.sh

Revert "exit with the exit code from a app when run()" (#71)

* Revert "exit with the exit code from a app when run()"