MPIExecutor wait_time always set by wait_on_start

[cchall]

The change to the wait_on_start feature of the MPIExecutor in 1ba9f57 to change it from bool to either bool or int looks like it results in MPIExecutor.fail_time always being ignored.

In the case wait_on_start=True it looks like MPIExecutor.fail_time should be used to set the wait time. However, the check

libensemble/libensemble/executors/mpi_executor.py

Line 169 in 2892c86

wait_time = wait_on_start if isinstance(wait_on_start, int) else self.fail_time

always passes with the first condition because bool is a subclass of int. This can cause failed simulations to not be caught if fail_time > 1 was needed.