Import bugfixes in settings validator. Hooks must be imported in validatore definition to avoid circular import failure

Implements both Plugin child classes handling extensions and session handling.
Modified the settings plugin preload piece of code. It is now adapted to the new plugins organisation.
Each Plugin child classes provides a confspec indicating where the plugin list is stored in configuration.

- now datasources are declared with two conf keys : read_only and identifier
- the old datasources plugin hook is now declared in lodel/plugin/core_hooks.py. This file is imported after lodel/plugin/hooks.py in the loader.py

- Added missing allowed_classes options to some references in em_test

- Modified again the em_test to fit with testing purpose
- First test on multi datasource query written
- debugging of query.py

Using the conf to load emtests.pickle

- I do not understand wy the set test for Settings was OK before...

- create_instance.sh create some symbolic links
- webui plugin that add webui configuration variable