Reduce global state and use `esmvalcore.config.Session` objects instead

[bouweandela]

In various places in the code, things depend on the value of esmvalcore.config.CFG instead of working with a esmvalcore.config.Session object. This may cause problems if users expect that the sessions work independently.

For example, changing the config_developer_file parameter in a session will change the esmvalcore.cmor.table.CMOR_TABLES global variable. It would be better to avoid global state as much as possible to avoid confusion and bugs.

The cleanest implementation seems to be only to use esmvalcore.config.CFG to start new sessions and not for any other purpose. The sessions would then need to be passed around to functions that need access to configuration. Modifying esmvalcore.config.CFG or the sessions should not have any side effects. esmvalcore.cmor.table.CMOR_TABLES will need to be removed.