MDOFunction.check_grad: reuse enums?

For the method argument:

    def check_grad(
        self,
        x_vect: ArrayType,
        method: str = "FirstOrderFD",
        step: float = 1e-6,
        error_max: float = 1e-8,
    ) -> None:
        """Check the gradients of the function.

        Args:
            x_vect: The vector at which the function is checked.
            method: The method used to approximate the gradients,
                either "FirstOrderFD" or "ComplexStep".

How about using this enum from derivation_modes.py:

class ApproximationMode(StrEnum):
    """The approximation derivation modes."""

    COMPLEX_STEP = "complex_step"
    """The complex step method used to approximate the Jacobians by perturbing each
    variable with a small complex number."""

    FINITE_DIFFERENCES = "finite_differences"
    """The finite differences method used to approximate the Jacobians by perturbing each
    variable with a small real number."""

This would change the API since from "FirstOrderFD" to "finite_differences" and from "ComplexStep" to "complex_step".