Directly check if two figures returned by a function are equal

[weiji14]

Check two figures (generated using slightly different methods) returned by a test function directly, similar to matplotlib's check_figures_equal function. Open to suggestions on names and implementation details.

Option 1

Original idea which is to follow matplotlib's style (see https://matplotlib.org/3.3.1/api/testing_api.html#matplotlib.testing.decorators.check_figures_equal):

@pytest.mark.mpl_check_equal
def test_check_equal(fig_test, fig_ref):
    fig_test.subplots().plot([1, 3, 5])
    fig_ref.subplots().plot([0, 1, 2], [1, 3, 5])

Option 2

Probably easier to implement, and we can reuse the same @pytest.mark.mpl_image_compare decorator, but there might be compatibility issues from some users?

@pytest.mark.mpl_image_compare
def test_check_equal():
    fig_test = plt.figure()
    fig_test.subplots().plot([1, 3, 5])

    fig_ref = plt.figure()
    fig_ref.subplots().plot([0, 1, 2], [1, 3, 5])

    return fig_test, fig_ref    

Option 3?

Some mix of the above two? Maybe another alternative solution?

Will resolve #94

[dopplershift]

I like option 1 above. I haven't reviewed the implementation in detail, but I like the concept in general.

[weiji14]

Hiya, I've actually ended up implementing Option 2 in this PR since it was easier (and more explicit), and also because I couldn't work out how to have two different marks (@pytest.mark.mpl_check_equal and @pytest.mark.mpl_image_compare) in the same pytest plugin. Option 2 is also less matplotlib specific (see GenericMappingTools/pygmt#590), but yes, I think we can discuss about how we want to go about with implementing this feature 😄