message: use std::fabs. Compilation issue on Fedora25
diff: #include <utility>
  #include <string>
  +#include <cmath>
  namespace Gudhi {
  namespace Persistence_representations {
  @@ -39,7 +40,7 @@ double epsi = 0.000005;
  * the top of the file. Setting up the epsi gives the user a tolerance on what should be consider equal.
  **/
  inline bool almost_equal(double a, double b) {
  - if (fabs(a - b) < epsi) return true;
  + if (std::fabs(a - b) < epsi) return true;
  return false;
  }
  