Adds set_zlim() method

[francocipollone]

Summary

Adds set_zlim() method which allows setting the minimum and maximum limit in the z-axis.

Example of use:

...
  plt::plot_surface(x, y, z);
  plt::set_zlim(-5., 40.);
...

[bingjingshidelei]

Can you add a function to set the scale value of the Z-axis