creating parent dirs for output_dir

[shimwell]

Currently when setting the openmc.deplete.CoupledOperator().output_dir to a directory with a subfolder the subfolder does not get created and the python script crashes with an error.

For example, this would reproduce the bug

operator = openmc.deplete.CoupledOperator(
    model=model,
    normalization_mode="source-rate"
)
operator.output_dir = 'outputfolder/anotherfolder'  # this path contains a sub folder

This PR attempts to fix the error by adding parents=True to the folder creation command in openmc.deplete.abc.py

[paulromano]

Simple enough -- thanks @shimwell!