Exploiting the linking propensity of a water encapsulated POV cluster anion [As6V15O42(H2O)]6- towards lanthanide cations: Syntheses, crystal structures and thermal properties of [{M(H2O)7}2(As6V15O42(H2O)]·xH2O (M = La3+, Sm3+ and Ce3+; x = 12, 13 and 11, respectively)