This thesis is submitted in accordance with the regulations for the Doctor of Philosophy degree at the University of Szeged. The results presented in the thesis are the author's original work (see Publications) with the exceptions of Section 1.1, which reviews some pre-existing material, and Subsections 4. 2.2, 4.3.1, 4.3.3, 4.3.4, 4.3.5 that contain results obtained by B.G. Pusztai. These are included to make the exposition self-contained.The research was carried out within the Ph.D. programme "Geometric and fieldtheoretic aspects of integrable systems" at the