add checkType and setType

Add checkType to know the different type and setType to set the type of
a variable in abstraction. /!\ we have to check the if and stuff like
that to know what to do when we have some Term and not only one (check
the Types)
Merge remote-tracking branch 'origin/master'

Conflicts:
	3-typed/src/main/scala/fos/Terms.scala