Add flatten argument to to_numpy in xarray wrappers

[JamesVarndell]

Is your feature request related to a problem? Please describe.

Many earthkit-data objects implement a to_numpy() method that accepts a flatten keyword argument. In other earthkit libraries, like earthkit-plots, we sometimes pass the flatten kwarg when calling to_numpy, depending on the use case.

However, the xarray Dataset and DataArray wrappers within earthkit-data do not support a flatten argument. This inconsistency can lead to errors when flatten is passed to their to_numpy methods. (See ecmwf/earthkit-plots#67).

Describe the solution you'd like

• Update the xarray Dataset and DataArray wrappers in earthkit-data to accept a flatten keyword argument in their to_numpy() methods.
• If flatten=True, reshape the resulting NumPy array to be 1-dimensional after converting with to_numpy().
• If flatten=False (default), maintain the current behavior and return the regular array.

This will ensure that all to_numpy() methods across earthkit objects have a consistent API and behavior.

Describe alternatives you've considered

One alternative considered was handling this issue directly within earthkit-plots by wrapping calls to to_numpy() in a try-except block. If an error occurred due to an unexpected flatten argument, earthkit-plots could retry the call without the flatten parameter.

However, this approach felt suboptimal for several reasons:

• It would hide inconsistencies between earthkit-data object types rather than addressing the root cause.
• It would make the code more fragile and harder to maintain, as future changes to to_numpy() behavior across different objects could introduce subtle bugs.
• It would introduce unnecessary complexity into earthkit-plots, which should ideally be agnostic to these internal differences.

Additional context

ecmwf/earthkit-plots#67

Organisation

ECMWF

[sandorkertesz]

Fixed by #685