Route dipole-gradient nep invocations through CALORINE_NEP_COMMAND

This is a follow-up of !249 (merged) which introduced an environment variable for pointing at the nep executable.

get_dipole_gradient/_dipole_gradient_nep/_predict_dipole_batch hardcoded the literal nep command instead of resolving it via calorine_getenv('NEP_COMMAND'), unlike batch_predict_properties and GPUNEP. This made the nep-backend dipole gradient unusable in any environment where the executable isn't literally named nep on PATH.

Default nep_command to None and resolve it via CALORINE_NEP_COMMAND at call time. Update the affected tests (test_nep_nep.py, test_nep3_execution.py, test_nep4_execution.py) to stop hardcoding 'nep' themselves.