CorrelatedFieldMaker: offset parametrization in one place

Hi @parras, @pfrank, @phaim,

in the CorrelatedFieldMaker as it stands at the moment, the offset from zero is parametrized in two separate places.

I propose to unify the offset parametrization into CorrelatedFieldMaker.make via the parameters offset_mean, offset_std_mean, offset_std_std.

While at it, modified the getting_started_3 demo (where the CF model is introduced first) to pass the parameters by name and with comments, giving the reader a chance to understand what is happening and providing a nice template for copying. Also, I renamed a few function parameters in CorrelatedFieldMaker in hope to make them more descriptive, but you might disagree with my judgement there.

As the creators of the model, what do you think about these changes?

Cheers, Lukas