Should we keep FASTEntity?

[jecisc]

I checked multiple FAST project and they do not seems to use this entity. It appears to be dead code.

In general the models are using FASTTEntity trait but do not subclass this class

[NicolasAnquetil]

I think it is important to have a common super class for all FAST entities
(same as what we have for all Famix entities)
So keep FASTEntity, make it super-class of all FAST entities, and remove FASTTEntity (see #102)