Issue title: option extent type in layer XYZ. 
 Issue body: I found a minor bug.
The extent option in layer/XYZ was tested as follow.
extent: [A,B,C,D] ----> not ok
extent: [[A,B],[C,D]] -----> ok

The extent option in others(Globe, terrain..) was ok in all of two types.    
 Comments: 
 Comment 0: I think it should be both available signatures.
{'source': 'GithubIssues', 'repo_name_with_owner': 'openglobus/openglobus', 'issue_id': 1076412861, 'repo_stars': 407.0}