ase.io.xsd: Added write function, fixed bug in read

Writing function for materials studio file (*.xsd) added for both periodic and non-periodic system. Read function assumed the file is non-periodic. This is fixed and the read function, now, can differentiate between nonperiodic and periodic materials studio file.

Note: materials studio is not capable of partial periodic system. (i.e. periodic in one dimension). When the atoms object is partial periodic, the write_xsd definition assumes it's periodic in all dimension.