Add functionality for the fpsemi-examples presentations

[MTWhyte]

Note: This PR is on top of #84

This PR adds functionality for the fpsemi-examples presentations, addressing issue #83. It is not entirely finished, but in a state where @james-d-mitchell and I might discuss finishing touches. This might include:

• I haven't yet documented addition on the author values, since I'm not sure how this should look.
• There is a bug, I think possibly related to setting default author values, which I'll describe in a comment below. I've kept the default author value for symmetric_group, but not set this up for other functions yet. I'll do this after we've figured out what's going on.
• I will also double check everything's in order (mainly the documentation) before marking this as ready for review.

[james-d-mitchell]

The CI will fail until we release a version of libsemigroups including the functionality that you're exposing here, so that's expected! If this didn't highlight any issues with libsemigroups itself, then I can make that release early next week.

[MTWhyte]

The author value bug

Where author values are given as the sole argument to these functions, things sometimes break (as in, Python freezes for a while, and then gets killed). This is not the case for every function. Playing with it now, it appears to be the as @james-d-mitchell suspected, with author values being interpreted as integers. The following is evidence for this:

In [1]: from libsemigroups_pybind11.fpsemigroup import brauer_monoid, author

In [2]: author.Carmichael
Out[2]: <author.Carmichael: 4>

In [3]: author.Machine
Out[3]: <author.Machine: 0>

In [4]: brauer_monoid(author.Carmichael) == brauer_monoid(4)
Out[4]: True

In [5]: brauer_monoid(author.Machine)
[1]    79396 segmentation fault  ipython

(The segfault with being since in libsemigroups, fpsemigroup::brauer_monoid takes a size_t argument).

[MTWhyte]

The best __repr__ for the authors would be to ditch the associated number, since that shouldn't really be relevant to the user. I tried to define this but wasn't successful, so that's also something for the todo list.

[james-d-mitchell]

@MTWhyte should be possible to take this to completion now!

[james-d-mitchell]

A few minor changes, then I'm happy to merge, thanks @MTWhyte