Modified the Objective class in saltelli.py so that fM_1 and fM_2 are
always 2D ndarrays and fN_j and fN_nj are always 3D ndarrays, even if
the objective_func only returns a single value. Previously, the shape of
the ndarrays depended on whether the objective function returned a
single value or an array of values. This made it very difficult to write
general code to handle the output of Varsens.