Change signal of Bz component in random_directions

[YagoMCastro]

Harmonica's function vec_to_angles inverts the upward compontent (Bz component in our case) so that the inclination points downwards, as bellow:

inclination = np.arctan(-magnetic_u / horizontal_component)

As Magali proposes that Bz component points down, it should be inverted when used in vec_to_anglesso that it keeps pointing in the right direction.

Suggestion

_, directions_inclination, directions_declination = hm.magnetic_vec_to_angles(
    x_r, y_r, z_r
)