Add mathematical formulation of the flatland problem to flatland book

We should consider adding a mathematical formulation of the flatland problem to the flatland book, maybe like in the thesis of Ryzner (see https://dspace.cvut.cz/bitstream/handle/10467/87776/F3-BP-2020-Ryzner-Filip-BP_FILIP_RYZNER_2020.pdf on page 8). I am not sure that all the details there are correct though.