Class MultiFileWriter

Writes structres to basename-NNN<extension> while never 
writing more than 100,000 structures to a file.

Options:
  basename - The base name of the written files
  extension - The extention of the written files (default ".maegz")
  sts_per_file - Maximum number of structures to write to each file (default 100000)
  
Files will be named <basename>-NNN<extension>

Usage:
  writer = MultiFileWriter(basename, ".maegz")
  for st in sts:
      writer.append(st)
  writer.close()
  files = writer.getFiles()

You may want to get basename using self.getOutputName(position)

NOTE: The writer must be closed after the last structrue is written.