cmake export

Closes #4

See merge request !2

libcerf.pc: remove hard-coded 'lib/'

See merge request !3

  - Added widthtest to test voigt_hwhm for numerous ratios gamma/sigma
  - Use assertion in voigt_hwhm to check for impossible situations
      (to avoid fprintf and exit, as suggested by Etham A Merrit)

Bug reported and solution provided by Alexander Grund, TU Dresden. Thanks!