Number of plates default should be ~50

[DaWiTH]

I suggest to set the default number of plates for the simulaton to values around 50. As the earth has around 50 plates, this number result in a more realistic world map, regarding the earth we live on.

I assume the main scope of new users is the usage of worldengine for creating realistic maps for e.g. games, or simulations on the rise and fall of cultures. By changing this number new users would faster see the benefit of this project. And for all other more specialized applications the number is still parametrized and thus is set to the appropriate number.

[sara-02]

@DaWiTH is this the file you are referring to change? https://github.com/Mindwerks/worldengine/blob/master/worldengine/plates.py#L16