950 load plugins in a separate namespace

[jesper-friis]

Description

Evaluate Python plugins in fully separate namespaces. This will hopefully get rid of the long-standing issue with possible interactions between plugins. This issue became even worse when we started to evaluate the plugins using the calling interpreter.

Type of change

• Bug fix & code cleanup
• New feature
• Documentation update
• Test update

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?

[francescalb]

Nice. I only wish for a test. Suggestion: import pyyaml, run dlite with a plugin so that the the yaml plugin is loaded. Do something with pyyaml.

[jesper-friis]

Nice. I only wish for a test. Suggestion: import pyyaml, run dlite with a plugin so that the the yaml plugin is loaded. Do something with pyyaml.

Added test_isolated_plugins.py