Modify sligthly CifWriter to handle material structure data stacking in one file

[E-Veillon]

Could it be possible to change a bit the CifWriter.write_file() method to make it possible to use append file mode 'a' or 'at' ? Like this it would become possible for projects handling high throughtput treatment of cif data to write a list of structures concatenated in one CIF file.

[janosh]

#3399 will be available in the next release.