Add hardcoded reference image test once API is stable (i.e. in conjunction with results manuscript, v1.0.0)

[sgbaird]

Something along the lines of the following (unfinished):

def test_png2xtal_from_loaded_images():
    # TODO: implement this once API is stable (i.e. 1.0.0+)
    imgs = []
    with Image.open("tests/V4Ni2Se8,volume=243,uid=8b92.png") as img:
        datum = list(img.getdata())
        imgs.append(img)
    with Image.open("tests/Zn8B8Pb4O24,volume=623,uid=b62a.png") as img:
        datum2 = list(img.getdata())
        imgs.append(img)

    data = [datum, datum2]

    xc = XtalConverter()
    decoded_structures = xc.png2xtal(imgs)
    assert_structures_approximate_match(
        example_structures, decoded_structures, tol_multiplier=2.0
    )
    return decoded_structures

See #146 (comment)