Plot other dset types

[DBAnthony]

In the config, example usage is:

other_dset_types:
    Total Prefit:
        regex: '(.*Prefit.*)'
        plot_ratio: True
        colour: 'red'
        add_error: False
    Total Postfit:
        regex: '(.*Postfit.*)'
        plot_ratio: True
        colour: 'k'
        add_error: False

To yield

(currently errors on ratio are plotted even if error on main ax is off)