adding expected threat (xT) example.

[andrewRowlinson]

Adding an expected threat example.

[andrewRowlinson]

@karunsingh Hi this the expected threat pull request. Grateful if you could take a look. It doesn't seem to converge quickly as in the blog post. When it finally converges the numbers are a lot higher than in the blog post (similar if run for only a few iterations).

[andrewRowlinson]

Here is the docs built for this example: https://mplsoccer.readthedocs.io/en/feature-xt/gallery/pitch_plots/plot_xt.html#sphx-glr-gallery-pitch-plots-plot-xt-py.

I'm planning on adding more text to describe it when I get it work.