Add check for .csv ending files that appear tab-separated

[alyssadai]

The current error message when this happens states that expected columns are missing, which is a bit misleading / confusing. We can try to catch and let the user know when the file ending and file separator appear incongruous.

Can borrow code from: https://github.com/neurobagel/bagel-cli/blob/16cf4b7860a2047c32116d280e5b6493a9be8d9a/bagel/pheno_utils.py#L42-L70

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