Add "thirdcircle" option to allow narrowed hemicycles

[RichardMN]

calc_coordinates can now handle segment values other than 1 and 0.5, making a symmetrical narrowed cycle. parliament_data now takes "thirdcircle" as an option to create a 1/3 pie cycle. parliament_data now also gives a more informative error for non-null incorrect type First vignette has an example of such a parliament, using data from the Australian senate. Documentation has been rebuilt (but this was largely a function of running checks).

[zmeers]

Apologies @RichardMN I forgot to tag you in my review - feel free to merge if you like. It's a substantial enough contribution to add your name to the author list as well, if you want.