Add interception storage decision

[CyrilThebault]

Summary

This PR adds a new model decision for canopy interception, with two available options: no interception and a GR5H-like interception reservoir.

The interception process is implemented as a discrete update before the soil physics. The interception store is maintained as a persistent model state but is not included in the ODE state vector.

The PR includes:

• new model decision: INTRC
• two interception options: gr5h_intrc and no_intrcep
• interception parameter MAXSINT_0 (maximum interception storage capacity)
• interception fluxes: incident precipitation (PIN0), throughfall (PTHRU), and evaporation from the interception store (EVAP_0)
• PET partitioning between canopy interception and soil evaporation
• integration into model outputs and parameter handling

Validation

The implementation was validated at the daily timestep.

For identical model structures and parameter values:

• INTRC = no_intrcep reproduces previous results
• INTRC = gr5h_intrc reproduces the reference implementation

Notes

• The interception store is intentionally implemented outside the ODE solver to avoid timestep-dependent dynamics.
• The differentiable mode has not been extended to support interception in this PR. Attempting to use interception with the differentiable mode currently returns an explicit error.