Prepare Version 1.1

[mmaelicke]

@rhugonnet,

I think it is time to publish version 1.1 soon. There have been some major improvements, like the sum of models just merged as well as cross-variograms. I will use this issue to collect stuff that should be merged / solved, before 1.1 is released.
Feel free to add stuff you want to be finished.

PRs:

• GStatSim interface #155
• Skip GSTools test in test_variogram if not installed #162

Issues:

• Update GSTools v1.5 interface #150