Fix for issue #22

[janlisec]

* Rdisop-1.67.5
* fix for issue #22; implementing atomic masses with higher precision
* the new set of elemental definitions will be used as the default which can
  be considered a breaking change as it might lead to minor changes in 
  previous analyses; previous results can be obtained by setting `method = 'IUPAC'`
  in function initialize

[sneumann]

Hi, looks good to me, thanks Jan for the PR ! Yours, Steffen