Changes to 1.0.20

* Add more sizes of icons including SVG, and add Keywords to the .desktop file
* Documentation updates
* Fix PeriodicExtension function
* ErrorFunction (erf) is using MPFR for real values so it is far more precise
  and faster
* The Fourier series example is no longer using a hand coded graph and hand
  computed series, so it can now be easily modified to show series for
  different functions
* Fix GCC 5 compilation
* A couple of minor fixes in the plotting code including one possible crasher
* Translation updates (Wolfgang StÃ¶ggl, Marek ÄŒernockÃ½, Maria Mavridou,
  ÐœÐ¸Ñ€Ð¾ÑÐ»Ð°Ð² ÐÐ¸ÐºÐ¾Ð»Ð¸Ñ›, Miguel RodrÃguez NÃºÃ±ez, Daniel Mustieles, Piotr Drag,
  Rafael Ferreira)

* During making of these changes the author (Jiri) was partially supported by
  NSF grant DMS 1362337 and the Oklahoma State University