[BUILD] do not warn about unused functions

- Wunused-function is a concept with very limited utility and it will
  warn about all files that include a header with static functions but
  does not use _all_ of them.
- for example: ITRAQLabeler.cpp includes MatrixUtils.h and uses the
  function convertOpenMSMatrix2EigenMatrixXd but does not use
  matrixIsIdentityMatrix -> a warning will be emitted about the unused
  matrixIsIdentityMatrix function.
[FIX] removed unused typedefs
[FIX] add static cast (no narrowing conversion)
Merge pull request #834 from hroest/fix/fixWerror

Fix/fix werror