Write sav files

[JanMarvin]

• [x] vtype needs to be expanded to add some -1 for strings longer than 8 chars
• [x] names need to be expanded for >255 chars will require longnames handling
• [x] var.labels implementation
• [ ] missings need handling kinda work already. could provide a missing table, though not sure if really required or beneficial
• [x] value labels need handling
• [x] long value labels need handling they are not long labels, just labels for long strings. nothing useful for the R world

[JanMarvin]

• [ ] compress = 1
  • [x] handle val_b and doubles in chunks and store them in a fashion similar to SPSS
  • [x] handle splitted character variables
  • [x] handle buf_s similar to buf_d
  • [x] improve this spaghetti code?
• [ ] handle strings longer than 255 characters