MAINT: Add a has_key method to BarData for compatibility.
MAINT: Add a has_key method to BarData for compatibility.

BarData should, at least for the time being, be compatible with
existing algorithms that had worked against the prior usage of
an ndict as data, which provided `has_key`.

Of note, the Python language has deprecated `has_key` in favor
of using `in` and `__contains__`.

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