Add missing method in seaice_drift.py

[sloosvel]

Description

Diagnostic script seaice_drift.py is crashing in the tests for release of version 2.13.0 due to a missing method in a class. This PR should fix it:

INFO:esmvaltool.diag_scripts.shared._base:End of diagnostic script run

• Related to Recipe test results for v2.13.0 #4157
• Link to documentation:

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[jlenh]

Perfect, thanks a lot Saskia and enjoy your break :)