Issue title: Integrate KernelUtils into Kernel
 Issue body: `KernelUtils` provide a set of methods for creating specific kernels. Those static methods should instead be moved to the `Kernel` class itself.

This change is part of the API review (see issue #11)
 Comments: 
 Comment 0: Done as of commit 140d063ac425bb441a353cacaa9750ef2736ccf2.
{'source': 'GithubIssues', 'repo_name_with_owner': 'eseifert/gral', 'issue_id': 67202938, 'repo_stars': 191.0}