Putting sourceAnchor on associations should not be generic, but per MM about famix

[anneetien]

There is no FamixJavaAssociation.
So do we want to put it on FamixTInvocation, FamixTInheritance, FamixTAccess and FamixTReference and remove it from FamixTAssociation?
@badetitou @NicolasAnquetil

[NicolasAnquetil]

Does not seem to make much sense to put it on the 4 FamixTInvocation, FamixTInheritance, ... instead of their super-trait
if we remove it, then remove it for good and add it only in the language MM
(and this is easy, just use TSourceEntity)

But I would also like to be remembered the initial reason why we might not want sourceAnchors in association ...
@badetitou do you remember why the issue was created ?

[badetitou]

I think we can close this exception. I have no use case for now. And nobody remembers the deep reason.

I suppose it was if someone want to define a new kind of association, but do not need the source anchor.
For example, an association to represent the link between two widgets in the Casino project?