(IrUtil) Remove a useless method
(EcoreUtil) Simplify signature for getEntity()
(ActorBuilder) No more nature migration needed
(ActorBuilder) Build dependent entities only for units
(ActorBuilder) Use a unique resourceSet as member
(ActorBuilder) Add comments, minor refactoring
(ActorBuilder) On clean, do not rebuild automatically

According to user preference, a FULL build is triggered only
if "Build automatically" option is checked in eclipse
(ActorBuilder) As most local variables set to final as possible
(ActorBuilder) Building dependency when needed

Actors are automatically rebuilt if they import something from
a Unit, if this Unit has just been built in FULL mode
(frontend) Delete IR file when needed

IR file corresponding to CAL file can be deleted in 2 cases:
1. The cal file is deleted from workspace
2. CAL file contains errors. This resolves #9
(validation) Check if instance is valid before building

The error message displayed is correct if an IR file is missing, for
example if there is errors in the cal file
(frontend) Ir files are built in the truth output folder

Fixes #5. Now each time an IR file is serialized from an Actor/Unit,
the file is generated in the output folder corresponding to the project
where the CAL file live. To achieve this, we had to store the project
of AstActor/AstUnit in an attribute of the corresponding Actor/Unit
(frontend) Clean useless methods

The Frontend.serialize() method has been removed. It was useless since
outputFolder and resource set can be computed at build time, instead of
stored in the false singleton Frontend
(frontend) Improve build of dependencies
Merge branch 'frontend_optimizations'

The merged commits fixed 2 issues and try to make frontent more efficient in general.
Some work still needed to speedup IR building and conusme less memory.