Crystal structure at 300 and 105 K, magnetic properties and Moessbauer spectra of bis(triaquatris(4-ethyltriazole-N1)iron(II)-N2,N2',N2")iron(II) hexakis(trifluoromethanesulfonate). A linear, trinuclear iron(II) compound, showing a unique high-spin-low-spin transition of the central iron atom