Experimental and theoretical study of a new C10N8 compound. Reversible photoisomerization between 2,3,6,7-tetracyano-1,4,5,8-tetraazanapthalene and 4,5,9,10-tetracyano-1,3,6,8-tetraazacyclodeca-1,2,4,6,7,9-hexaene in low-temperature rare-gas matrices