Outdated docs and tutorials about importing Client/DBClient

[seisman]

Some docs and tutorials use the following code to import the client,

from mspasspy.db.client import Client as DBClient

from mspasspy.db.client import Client

but now it fails due to the recent renaming in 80fff8868e8f26614eaf1a10da3f9a6429e7f9cb.

Need to update the docs and tutorials to use

from mspasspy.db.client import DBClient

[wangyinz]

Thanks for catching it. We will revise all the documents when the API is finalized.