Lanthanide(III)chloride-tetrahydrofuran solvates: structural patterns within the series LnCl3(THF)n, where n = 2,3,3.5 and 4: crystal and molecular structures of [PrCl(μ-Cl)2(THF)2]n, [Nd(μ-Cl)3(H2O)(THF)]n and GdCl3(THF)4