Changing w_max should also change ft_mask

[gustavdelius]

The ft_mask is used when calculating predation rates using the Fourier transform method and is supposed to mask out any contributions outside the fish size range, i.e., anything larger than w_max. Currently ft_mask is only set when a MizerParams object is created by emptyParams() and is then never changed, even when the user changes the w_max species parameter.