* add rigaku_dat sample files
* is Rigaku RIG the same as rigaku_dat?   
* implement GSAS format (see gsas.cpp for comments) 
* PANalytical XRDML
* consider adding JCAMP format (Standard NMR Data Format)
* allow compressed (.gz, .bz2) data files (only in standalone version,
  if Boost.Iostreams headers and libbzip2 and/or zlib are present)