Refactor: manage Julia and membrane loaders at the level of source code generators instead of globally in JuliaConfig.

In optimization level source code generators, separate the setting of the reference to the Juliac instance from the initialization process.

containsKey method added in MembraneLoaderItf to test whether a controller descriptor is registered.

Bug fix in the Juliac MOJO to enable using it with Clover (issue reported by Nicolas Salatge from Petals Link.)

Documenting TODO for Juliet.
Anticipate next version number: 2.3 instead of 2.2.7 due to the API refactoring that has been performed by the last commits.

Remove uneeded protected modifier.

Refactoring: Towards a better modularization of the retrieving of configuration data from .cfg files.

Typos.