stable_timestep(aquifer) can give large negative dt value

[verseve]

Wflow version checks

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Reproducible Example

The output of stable_timestep(aquifer) can get negative (large value), starting with a cold state.

Current behaviour

The wflow model type sbm_gwf gives the following warning:

┌ Warning: stable time step dt -839465.3444686249 for groundwater flow is smaller than sbm dt 1.0

Desired behaviour

dt for groundwater flow should always be a positive value.

Additional Context

No response