splat.data.Fragment: add support for saf files

* add support for Splat Audio Fragment 'saf' file format
  which is a Splat specific format with all the sound
  data directly compatible with a Fragment (i.e. 64-bit
  floating point or 'double')
* fix issues with file extensions and explicit file format
  identifier in Fragment.save and .open
* add saf case to test_frag_save