Added pandas_ndim FUTURE flag

[trexfeathers]

🚀 Pull Request

Description

In preparation for breaking changes to be made to iris.pandas.as_dataframe() and iris.pandas.as_series() - #4669.

QUESTION: how should future flags be documented? The existing datum_support flag is not documented but that might just be an oversight?

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Consult Iris pull request check list

[rcomer]

how should future flags be documented?

Example from Ye Olden Dayes https://scitools.org.uk/iris/docs/v1.13.0/iris/iris.html?highlight=future#iris.Future