Extract plot method

[wleoncio]

There is one (and perhaps only one) function, Exact_unconditional_test_2x2(), that contains code for plotting. It's currently hard-coded to not show the plot, so it may be better to separate this code into a plot() method, either for the contingencytables_result class or for a subclass of it. See code below for relevant chunk.

https://github.com/ocbe-uio/contingencytables/blob/6355e889e7c2e2c9550e7e1cb0c6cdbd33a224a2/R/Exact_unconditional_test_2x2.R#L112-L121

[wleoncio]

There are actually two functions: McNemar_exact_unconditional_test_paired_2x2() also had this, but was removed on 54c2cfe.