CovModel.opt_arg should be sorted

[MuellerSeb]

GSTools/gstools/covmodel/base.py

Line 166 in 67a228d

self._opt_arg = list(opt_arg.keys())

In order to have a unique representation for an argument list, the optional arguments CovModel.opt_arg should be sorted by name.
This should be done in CovModel.__init__ at the shown line of code.