Update preprocess.py

[szmazurek]

This is in the reference to the nice Python "suprise" that is mentioned in TODO - when we want to have boolean flag in argparse args, we should use store_true or store_false action flag as in other cases indeed all input strings will be evaluated to true

Fixes #ISSUE_NUMBER

Proposed Changes

Checklist

• [ ] CONTRIBUTING guide has been followed.
• [ ] PR is based on the current GaNDLF master .
• [ ] Non-breaking change (does not break existing functionality): provide as many details as possible for any breaking change.
• [ ] Function/class source code documentation added/updated (ensure typing is used to provide type hints, including and not limited to using Optional if a variable has a pre-defined value).
• [ ] Code has been blacked for style consistency and linting.
• [ ] If applicable, version information has been updated in GANDLF/version.py.
• [ ] If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file.
• [ ] Usage documentation has been updated, if appropriate.
• [ ] Tests added or modified to cover the changes; if coverage is reduced, please give explanation.
• [ ] If customized dependency installation is required (i.e., a separate pip install step is needed for PR to be functional), please ensure it is reflected in all the files that control the CI, namely: python-test.yml, and all docker files [1,2,3].

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